Would a community letter be appropriate for this?
To what purpose? Mongoose reads these forums and is aware of our opinions. This community is a small fraction of the total Traveller player base. And even within that small community, not every one agrees that this is a problem.
And even if you accept that it is a problem, there is lots of ways to fix it. They need a new ground combat system anyway, so they could write one that isn't so "may the best TL win" so that the statement about being equal to the Zhos is actually true.
Or they could clarify that the Imperial Army is, in fact, a patchwork of militia units that, on average, are TL 12. So Vice Marshal Lord Bryor has his 25 divisions of TL 15 soldiers and the reinforcements are Porozlo's 75 divisions of TL 7 troops. And the standing "Army" is Headquarters units that are trained at integrating the various logistics and command & control elements of these disparate units.
Fuzzy isn't a problem. IMHO, it is that they make a big deal about it not being as advanced as it could be and then saying "But it's just as good as if it was that advanced." They say "totally not fuzzy, definitely this!" and then saying treat it like its fuzzy. They didn't have to say the army was TL12. They didn't give any stats for any mass combat system for any of these units, so what's the point of trying to give an overall TL instead of just discussing the gear and label it at the earliest TL it gets deployed it, like all other gear in Traveller?
I vehemently dislike the argument that everything needs to be nailed down and given a cast in imperishable crystal meaning. We have TL15 battleships being made obsolete by other TL15 battleships. There's no mechanics to support that and that's fine. TL15 is a range and you can imagine it having iterations without requiring there to be rules supporting that.
But if you do choose to nail something down, then don't make it wishy-washy. Same thing with world building. The UWP is intentionally flexible. But once you write up the details on that world, you do need to explain exactly what you mean and how that relates to the shorthand. So if you make a big issue of how logistics & politics force the Imperial Army to be second rate, then run with that. Or give more than half a sentence to how they manage to actually perform as well or better than the Zhos despite that.