This is different. Those internal registers are not accessible at all by software, they are part of the micro-architecture that emulates x86 to the rest of the system, and are actually used. Your modern CPU is like a virtual machine.

That Hitachi CPU however physically had those unused registers as well as that native mode where instructions wouldn't artificially be delayed to be cycle-compatible, but by default behaved like the older and slower model and was only advertised as such.

That doesn't make too much sense first glance, unless there was some glaring bug discovered after production started, or as suggested they feared it would make some more expensive "enterprisey" CPU from another lineup obsolete.