Yeah, their cache work is undeniably really cool.

That being said I think it's a natural consequence of the difference between a mainframe and commodity servers. A mainframe is going to be running pretty disparate workloads simultaneously, so it makes sense to steal from your neighbor if they aren't using their cache. Whereas it's more likely that a commodity server is just running the same server on each core, and if you have a different workload, you pick a different shape of server to run it on. There are pros and cons to both.

I do wonder about the spectre consequences of borrowing cache lines from other cores though.