Hey I am one of the guys who made this. Thanks for giving the game a try, we honestly had around 22 hours to make this from the ground up and really appreciate the feedback!.
As for the assets, none of it was ripped or copied, I drew and animated them myself; Yes they are based on the Hollow Knight game and characters but they weren't ripped. Upon seeing the theme, the idea of reversing Hollow Knight, as The Hollow Knight, was something we loved and just wanted to make it. Our goal was not to 'plagiarize' but to simply make something for fun and in a way pay homage to the great game.
As for it being valid submission, it is honestly up to the organizers of the jam. There are many submissions that has reversed an existing game and they have all been incredibly creative and fun. At the end of the day if the organizers invalidate this then oh well, in the little time we had to make this, we had fun with it.
The game mechanics definitely need some work to it. We are planning on completing it, fixing the bugs and add some additional features to create this into a standalone fan game.
Regardless, thanks for the trying out the game and leaving your feedback, we hope you like the finished product!
P.S: I did have to recreate the art as I used skeletal animations to animate, but I am quite happy that you think its good enough to have been copied straight from the game hahah.