Well that is a good question and confusing at best:
1. Grant of license
This Agreement is a license, not an agreement of sale. Licensee shall not acquire any copyright ownership or equivalent rights to any of the Licensed Content. Seller and the Licensed Content sources retain all right, title and interest in and to all of the copyrights, trademarks, and all other proprietary rights in the Licensed Content. All rights in and to Licensed Content not expressly granted in this agreement are retained by Seller or its suppliers.
Licensee may provide access to Licensed Content to no more than ten Authorised Users simultaneously, provided that access to Licensed Content is used solely for the purpose of creation or reproduction of Works for Distribution made by or on behalf of Licensee.
Licensee may use the Licensed Content in up to 1 commercial projects. A commercial project is one defined as a Work for Distribution launched with the capability to generate revenue, or intention to generate revenue through the sale of, licensing of, or otherwise intend to generate revenue directly from the Work for Distribution. If Licensee wishes to use the Licensed Content in more than 1 commercial projects they will need to purchase an additional License or the Licensed Content at the exclusive price if one is available (c.4).
Licensee is permitted to use the Licensed Content in unlimited free projects. A Free Project is one defined as a Work For Distribution that generates zero revenue, has generated zero revenue and does not have the capability to generate revenue in any form.
2. Restriction on Use
Licensed Content may not be used contrary to any restriction on use indicated herein.
Licensed Content may not be resold, sublicensed, assigned, transferred or otherwise made available to third parties except as incorporated into Works for Distribution. Licensed Content may not be distributed to third parties as standalone files or in a way that unreasonably permits the recipient to extract the Licensed Content for use separately and apart from the Work for Distribution.
Licensee may not distribute the Licensed Content in any library or reusable template, including but not limited to game templates, website templates intended to allow reproduction by third parties on electronic or printed products. Licensee may not distribute Licensed Content in a manner meant to enable third parties to create derivative works incorporating Licensed Content.
Licensee may not superficially modify the Licensed Content and sell it to others for consumption, reproduction or re-sale. For example, but without restriction, Licensee may not resell audio tracks as backgrounds, hold music, ringtones etc.
Licensee may not incorporate the Licensed Content into a logo, trademark or service mark.
Licensee may not incorporate the Licensed Content in advertisments (excluding advertisments for permitted Work For Distribution), broadcast TV, theatrical or movie releases.
Licensee shall not use the Licensed Content in a manner that violates the law of any applicable jurisdiction.
Licensee shall not claim copyright or attribution of Licensed Content.
Licensee shall not use the Licensed Content in a pornographic or defamatory manner, whether directly or in context or juxtaposition with other materials.
3. Term and Termination
The license contained in this Agreement terminates automatically without notice from Seller if Licensee fails to comply with any provision of this Agreement. Upon termination, Licensee must with immediate effect stop using the Licensed Content, destroy, delete and remove the Licensed Content from Licensee’s premises, computer systems and storage. Licensee must also make all reasonable efforts to ensure that copies of the licensed content are removed from any locations it has been distributed to.
4. Exclusive Licensed Content Pricing
If you purchase an asset an an exclusive price, you are also bound by all terms in this License (c). An exclusive price means that the item from that point onwards will be delisted from the Scirra store and no longer for sale. Non exclusive licenses may have already been sold up to that point. Use the sales figures published on the items page for indication of how many have been sold to date. Licensed Content purchased at an exclusive price are permitted for use on unlimited projects within the terms of this Agreement.
If you receive a refund for payment of Licensed Content in part or full that you requested, your license is terminated with immediate effect and you must follow all termination procedures listed in (c3). If you purchased Licensed Content at an exclusive price it may be relisted for sale in the store.
I don't see anywhere it limits you to any specific engine.
I am concerned that they use a boilerplate license on all the store assets because for example Kenney's sprites are royalty free and free to use in as many commercial or free projects you want but the store has slapped this license on those assets making it a legal conundrum if you acquired those assets direct from Kenney or another free source.
Unless Scirra is claiming copyright for all assets in the store (that would probably surprise people like Kenney) the license would probably not hold up in court as it must be signed by the copyright holder not a store they use to sell their assets.