I like the idea of using momentum for your jumps, this could be really cool if you had more time to work on it.
The controls work well, but it took me a few deaths to realize that I had to use the arrow keys to move. From your explanation, I thought pressing numbers would make me advance, so I was trying different numbers to move at first haha.
Rewinding the boss is cool, but it's actually possible to beat the game fairly easily without rewinding the boss, or even using the speed up button.
The game feels very unfinished, not because it only takes several seconds to beat (I actually enjoy that), but it feels unfinished because you can fall off the map so easily. Just moving forward at the start leads to falling off the map, and there's no reset mechanic for that.
However, feeling unfinished doesn't mean that it's not a "complete game." You submitted in time, and the game can be "completed," so congratulations on submitting your first game jam game.