That assessment is itself often made post-hoc, though. The dabbler who hits on an interesting connection by chance is often retroactively written into history as a generalist who purposely scoured the world far and wide for that knowledge to connect to their project (sometimes people even convince themselves that that's what they were doing all along).
Same with what makes one a "dilettante" versus "Renaissance man", I think: it's more or less a retrospective judgment, and the main factor is how much your output impresses people. The mediocre would-be Renaissance man is a dilettante, and the dilettante who does impressive things is a Renaissance man.
I agree, they're not that useful as descriptive categories for people who are still alive and working. However, I think they're good as prescriptive categorical goals/mindsets: if you want to be remembered as a generalist, then you should be striving to find connections outside the fields you're aware of, and then pursuing those connections where they lead.
Same with what makes one a "dilettante" versus "Renaissance man", I think: it's more or less a retrospective judgment, and the main factor is how much your output impresses people. The mediocre would-be Renaissance man is a dilettante, and the dilettante who does impressive things is a Renaissance man.