If that were the true explanation, I'd expect followup videos of cats freaking out just as much over other innocent objects -- it's what some people would try and others would upvote. I didn't see any when I first heard of the cucumber thing, so I'd guess that the phenomenon's real: cucumbers seem especially like poisonous snakes to cats. I won't try a controlled experiment on my own cat, though.

The opinions on quora don't appear to be backed up by anything more than 'not proven'. I think it's reasonable to be skeptical, but also that experts are still pretty ignorant and prone to dismissing phenomena they don't already know about.