You can't compare cores like that.
Take a look at the cache size on the Telum II, or better yet look at a die shot and do some measuring of the cores. Then consider that mainframe workloads are latency sensitive and those workloads tend to need to scale vertically as long as possible.
The goal is not to rent out as many vCPUs as possible (a busines model in which you benefit greatly by having lots and lots of small cores on your chip). The goal for zArch chips is to have the biggest cores possible with as much area used for cache as possible. This is antithetical to maximizing core density, and so you will find that each dual chip module is absolutely enormous, and that each core takes up more area in the zArch chips than in x86_64 chips, and that those chips therefore have significantly less core density.
The end result is likely that the zArch chips are going to have much higher single thread perf. Whereas they will probably get smacked by say a Threadripper on multithreaded workload where you are optimizing for throughout. This is ignoring intricacies about vectorizatiln and what can / can't be accelerated and whether or not you want binary or decimal floating point and other details and is a broad generalization about the two architectural general performance characteristics.
Likewise, the same applies for networking. Mainframe apps are not bottlenecking on bandwidth. They are way less likely to be web servers dishing out media for instance.
I really dislike seeing architectures compared via such frivolous metrics because it demonstrates a big misunderstanding of just how complex modern CPU designs are.