I said this is where it would help (AFAICS), not that it was the best solution.

These cores are typically licensed with class/restrictions so in absolute terms yes but in the financial engineering of how the system is delivered with excess and restricted hardware no (see core types on the prior/shipping generation here https://www.ibm.com/downloads/cas/6NW3RPQV)

There are probably design reuse and RAS considerations that make it not currently worthwhile to i.e. have a distinct physical design for SAP or whatever cores.

I wouldn't be surprised as well if there was some binning that occurred – the dies are huge, so why not overprovision in the design? (Although erring on the side of slighlty more surprise in the case of some binning, since IBM mainframes seem to exist beyond the laws of commodity economics, and it looks like they're using a 5nm node.)

the CPU's only appear to use about 1/3rd of the die area. Most of the space is cache.

People buy whole machines to run memcached!