"Pick all" is left from previous version, it's not needed here, you can remove it.
"For each chooser order by random(1)" is to pick two random chooser sprites (excluding the one we removed in event 5). It doesn't matter what number you put in random() expression in this case, it's just used to sort the list of instances randomly. Then "loopindex=1" condition is used to stop the loop after two chooser instances are processed. Again, this is not really needed, the loop can continue for the remaining chooser sprites, but Dino sprites will not be changed after that.
random() doesn't always need a range, random(1) means the same as random(0,1)