The game doesn't use hitboxes at all, it uses pixel-perfect collisions. Meaning, it registers a collision if two pixels are on top of one another.
Since the spear's head is small, you have less "area" to work with, but it is expected of you as a player to master the aerial controls (you will notice that in the air, the characters turns much slower than on the ground, which in itself also has a small delay. All of this is deliberate and mastering these controls is a skill the player is expected to develop).
Regarding the "impossibility" of the game... the game was balanced for mobile, which means it is significantly easier in the web version. To be more precise, when speedrunning the game, an experienced player can complete it in less than 15 minutes. A first time player is expected to require around ~2 hours to complete the game.