I kinda choose to somewhat believe in this hypothesis because it's mighty fun, and makes the past look so much more romantic!
But it's implausible for all kinds of basic reasons.
It's just clear all throughout history that humans do not have trouble making up fantastic creatures. There's no reason to think that these fantastic creatures weren't made up. It just seems at first blush that there might be a reason because it was "very long ago." Until you realize that "very long ago" by the standards of hindu myth are wildly different than "very long ago" by the standards of hominid species.
No, that is just ignorance.
There is enough specificity and detail in Sanskrit literature to easily refute this. See this comment of mine for details - https://news.ycombinator.com/item?id=50284725