There are some valid differences:
--538 shifted from 'polls only' to 'polls plus' as default. polls plus in '16 was clinton 73.5% on 8/26
--polling and priors now support a Trump E.C. edge, not yet true in '16
--Biden penalized post DNC https://twitter.com/gelliottmorris/status/1298797791523868673?s=20

So, for instance, the 538 polls plus estimate of PA/MI/FL on 8/26 in '16, v. today. Are they very different? No, not really.

There's a second question: how much uncertainty is there today v. 16? Here, FiveThirtyEight doesn't see much of a difference. TLDR: uncertain fundamentals cancels out fewer undecided voters.
Is that right? IDK, and tbh they don't either (one might say that's the uncertainty)

I think the strongest substantive critique of the FiveThirtyEight model is that it's leaving info on the table by omitting Trump approval. If you don't know what the fundamentals are--and I don't think we do, empirically--the approval at -10 might help sort it out.

That said, they didn't use approval in '16 and presidential approval is not exactly the same as the 'fundamentals,' either. It's so correlated with vote choice that you're getting close to modeling off of the dependent variable here