My theory on Chopper is that after taking the rumble ball so many times, Chopper has effectively changed his DF into something completely different.
The Hito Hito no mi simply allows the user access to 2 forms: human form and human hybrid.
By taking the rumble ball, Chopper was able to expand the number of forms he could use. At first he needed to take a rumble ball every single time just to use an alternate form. Post skip, he can change into most of his alternate forms without need of a rumble ball and he only needs one rumble ball for monster point.
Chopper having a god fruit from the start would be a stupid plot twist. It was an obvious retcon for Luffy but at least with that, Luffy is the MC so we always knew he was super special. Chopper is literally a mascot tho. Him changing his DF would be a much better avenue to explore.
We know DFs are tied to dreams. While Chopper presumably had the human human fruit, we never saw him develop his DF to make himself more human. Rather chopper went in the completely opposite direction, using his fruit to make himself a monster.
Through his own intention and the rumble ball, could very well see Chopper changing the hito hito no mi into the “monster monster fruit” or something