I don't think that's what defines a polymath, for if it does, then none existed. My view of what makes one a polymath is deep --likely cutting-edge-- expertise in several disciplines.
That being said, I agree with the general sentiment that it is increasingly harder to become a polymath these days, especially in the disciplines with lots of active research. To have an expertise in just one discipline it takes years of education (to catch up with progress) and then a considerable recurrent investment of time to stay up-to-date.
If one is to be a polymath in a varied set of disciplines where overlap is minimal (see Leonardo), one would have to go through the mentioned process of acquiring and maintaining expertise for each discipline. This is different for a polymath in a set of closely-related disciplines, because there's only so much more (compared to being expert in one discipline) one needs to do to be a polymath because there's a lot of overlap. But it is debatable whether that even counts as a polymath, a point raised in:
> Is Judith Butler’s supposed eminence in ‘philosophy, linguistics and politics’ enough to qualify her?