Yeah, it looks like they switched over to desktop chips around 2017 (they still use laptop memory though -- except for imac pro).
An i9-9900K has a 95w TDP, but Anandtech puts the real load number at around 170w TDP. I've seen people undervolt these down to around 110-120w in the 4.7GHz range. I imagine Apple's dynamic undervolting and custom motherboard can shave 10% or so off that total while their dynamic undervolting can go much lower with fewer cores and lower frequencies. While even that isn't going to make their tiny cooler keep sustained loads from throttling, it could get much closer.
By "laptop memory" you mean SO-DIMMs; the only significant difference to full-size DIMMs is, well, size. Voltage and frequency tends to be the same, leaving aside extreme overclocker's RAM.
In c't magazine's review of the current iMac, they found that the whole machine appears to have a power limit which is shared by CPU and GPU, so yeah, Apple are definitely doing something fancy in that regard.
> With the exception of the mac pro, Apple's entire lineup uses mobile processors.
In fact their entire desktop lineup now uses desktop-grade CPUs. (except possibly some entry-level iMacs that weren't subject to the recent refresh)