Actually, I do not know what you expected. First of all, GMS is unable to create dynamic sounds on his own, given that it doesn't even have an audio processing engine, so the only way to work with something like this is with pre-recorded audio. Second, it's a basic engine (stated in the post) which not only works with audio sync groups but it also controls things like decays and fade time. Third, the reason that this thing is paid it's almost exclusively because of the music (which is reusable in any project, only needing credits for composition), and finally, it's ready for any user to understand it quite quickly, given that sync groups are not a common theme in the tutorial scene for GMS. 95% of the code assets you can buy are actually pretty simple scripts or small engines made for the comfort of the more novice users.
That statement stated, I'm actually eager to see if you can give some tips to update this with some other things.