What makes you say that? I've worked for two spacecraft companies in the US who supply parts (and whole spacecraft) to NASA. Both of them did drawings for small parts in mils (thous) for both electrical and mechanical parts. It seems like most machinists in the US are used to working in mils.
I remember on my first day (year 2012), I was shown an E-sized drawing of a spacecraft bus at 1/10 scale. The unit was inches. I was surprised it was in inches and said something. The person showing me just laughed and said welcome to aerospace.
The mechanical specifications I've seen for newer (post-2000, say) components at the lab that built this particular robot have all been in metric units. I just checked, and the current requirement is that metric "shall" be used, unless (a) a particular mission waived that practice - I don't think MSL did; or (b) for parts from "industries that are historically in another unit of measure." Two examples I know of are gears and printed wiring boards, which are indeed in mils.
And you're right, it is possible, barely, that the wheels were an exception. I don't know if they were fabricated on lab or not.
