If you found another post, you can try to ping the original poster to update the sharing on dropbox. There was a change earlier this year that broke links, but the files are still there.
In your example, the problem is that the player moves with the platform when you change the platform's position (the player gets shifted 800px to the right, off the screen.
You can set player's position to self.x-800 (or -801? to compensate for your other events) in the same action when you reset the platform position. You'll also want to add a System-Wait 0 seconds action at the start of this event to prevent blinking.