They still aren't identical constructs, because the intended action is very different; multiple insertions result in multiple distinct records all with different values while multiple updates result in multiple records with the same new values

Having the syntax identical is inferior in every way except for the case where you want to build the string up programmatically, and that is a trivial problem to solve for the caller.

I think your proposal works better for an UPSERT keyword, as that is yet another different intended action.