For the record, my logic on this, and why it's not the same thing is that I see a distinction between a section of a route being poorly signed through neglect and a section of route being actively and deliberately unsigned.
UT 190's eastern extension to Guardsman Pass is a case of the former - there are no shields on that section of the road, but there isn't any signage stating or implying it
isn't part of UT 190 either.
The section of UT 201 on 2100S, on the other hand, is a case of the latter - signage for the exit to it from I-15 just says
"2100 S", and signage from the east end at US 89 says
"TO UT 201" - explicitly stating, by virtue of that "TO", that that section of 2100S is
not part of UT 201. It can be readily surmised by how things are signed that there is clear intent on UDOT's part to exclude 2100S from UT 201 as experienced by motorists. Indeed, it doesn't even directly connect to the rest of the route.
The general principle at play here is: if the signage is consistent (which is the case for UT 201), then the signage rules supreme if it disagrees with what exists on paper. However, if signage is inconsistent or missing (which is the case for UT 190), then we use what exists on paper as the fallback.
This is generally in line with how things have been handled elsewhere, but if anyone has anything else to say about it, I'm open to hearing it.