Obviously no one can prove that the poll is legitimate and that there aren't fake/additional accounts being used. However, any claim that it's illegitimate would be unreasonable, just because the amount of malicious people and hardcore pumpers is going to be roughly equal on each side.
Just like the F2Pool poll showed before, public support is largely for SegWit. Miners will have to follow eventually or nothing will happen at all.
I hope there will be a mining hash pool. The present poll can be rigged. Somebody could register many Twit accounts and rig the poll.