Bendy light was never proven to disprove!
You dont have to prove something before disproving it, e.g:
Theory: all sharks wear bow ties at all times.
Disproof: I can see a shark without a bow tie on, therefore it is disproved.
However the flattists are fond of non-falsifiable theories which cannot easily be disproved, an example with the shark metaphor would be:
Theory: some sharks wear bow ties some of the time
Attempt at disproof would require being able to observe every shark in the sea for the duration of its lifespan, therefore it is impossible to disprove.
Bent light is however a falsifiable theory and predicts certain visual effects which, if they do not happen, are the equivalent of the non-bow tie wearing shark.