I think casual polymath seems like the wrong descriptor to use - as they're not experts in other fields. They're experts in one field, and (most probably) dile

I think casual polymath seems like the wrong descriptor to use - as they're not experts in other fields. They're experts in one field, and (most probably) dilettantes in other fields. The trick however, is that they use their expert status from one field, to reach their audience. And then their followers will take that as domain authority, because they're perceived as authority in some other field. I'm not going to name names, but this certainly fits the pattern of a lot of people in the VC-industry. Not that they brand themselves as experts (sometimes they'll explicitly state that they're not experts) - but their followers will assume that it's expert knowledge.