I'm pretty sure that T-Mobile has plans to cover that area, in the same way that the US military has plans for the invasion of Paraguay. Somewhere, in some filing cabinet there is a plan for each gathering dust. Do either intend to act on these plans any time soon? Look at the evidence. T-Mobile has had a full suite of mid and low-band spectrum in this area for years, yet they have not acted to provide service to the area. They are building new coverage at a furious rate in other areas but not there.
In the past I've held my breath waiting for T-Mobile to provide service to some important areas. All that happened was that I turned blue.
I know the area very well and the map I was shown by an engineer was based on coverage from an AT&T site in town, not TMO signal. Whoever TMO hired to drive test the area and record signal levels screwed them! They added 6 new sites in El Dorado county in tourist areas that had no coverage before and along the Eastern Sierra, but North Amador isn't even on any build list. Probably because of the bogus drive testing. The current map is a little more correct, but every "verified call" in Plymouth just says "fair signal" but there is never anything worse. I think "fair" is their term for "very poor". I frequent the RV park there and if I leave my phone up in a tree branch outside, I can occasionally get a text through. If that is their definition of "fair" then they need to redefine.