[Really] my statement was a generalization. I felt it unfair to make the comparison as it was presented.That's not really an accurate characterization, as the Xeon line is very broad for a range of applications. Depending on which processor you select, you can find Xeons that have identical characteristics as specific Core i7 and i9 processors.
Those are the processors you chose for comparison [but] I would state that even when comparing similar-generation processors, you will find many instances where the conditions I stated exist... regardless of the difference in generations of processors.As for the single-core benchmark differential between the iMac Pro and new Mac Mini, that is probably best explained by the fact that the iMac Pro uses a Xeon based on the 6th-generation Core architecture (Skylake) and the Mac Mini uses 8th-generation (Coffee Lake), as well as the fact that the Mini can TurboBoost to 4.6 GHz compared to 4.5 GHz for the iMac Pro.
Yes, the Xeon family is fantastic and wide-ranging. Intel is also doing their best to blur the line between their desktop and workstation processor families in regards to clock speeds and number of cores. But in the end, you do not purchase hardware with Xeon processors for their single-core benchmarks. If that is your sole criteria, you would be better served by an i7 or i9, and overclocking them to their maximum levels. Rather, it is for the reasons I noted you go with a Xeon solution: where you really need 18 processor cores, or multiple multi-core processors, or ECC memory. And in those conditions you will leave the i-Series in the dust in regards to benchmarking.