The big flaw in uniform swing analysis is that environment changes the local context massively. Incumbents look a lot safer when they're not challenged, because if some random dude is running against you with no money or enthusiasm in a neutral year an entrenched incumbent will cruise to an easy victory. Saying that because you won reelection last time against a nobody by 25 points you need a uniform national swing of 25 to be taken out is total nonsense.
This year Democrats are, generally speaking, running solid, well-funded candidates pretty much everywhere. Of course, this is a result of a more favourable national environment but if that somehow happened in a neutral year it would by itself move the needle substantially in their favour. Add on factors like reduced gerrymandering, Republican retirements and less room for swing in already safe areas and uniform swing analysis totally misses the point.
