What you say is true, but apple has generally been better at selecting processors from intel that aren't just re-bagged low clock xeons. It wouldn't surprise me if this thing is 5Ghz turbo on a couple cores. There was that 28core 5ghz beast intel was talking about (definitely not 300W though) and a few rumored W3175X, follow on.
All of the Xeons in Mac Pro say 4.4Ghz turbo from what I saw at apple.com.
So the 2019 iMac i9 @ 5Ghz on 4 cores will beat the crap out of them for software development purposes, and from the benchmarks so far it seems there is very little (or none at all) throttling on new iMacs.
