The cognitive dissonance is often due to the mistaken idea that Traveller is only about the Third Imperium...
And for more than 3 editions that was in fact the status... MT, TNE, T4, and GT/GTIW.
For many CT players, especially post 1981, it was as much about the award winning setting as the award winning game engine.
There's no reason one can't do non-OTU games in MT, GT, or GTIW, but it wasn't called out as so.
TTNE did call it out, but only in FF&S...
T4 also called it out, but focused so much on supporting the OTU tech for the timeframe that it was hard to run the earlier editions' subsettings. (It's hard to run the classic 1107 OTU in the Solomani Rime when the book only covers to TL12.
For GT and GTIW, there's no reason to use them for non-OTU settings, except for understanding/integrating GT:FT. And it has several other low effort needed sci-fi settings already, including G:Transhuman Space, G: Prime Directive, G: Space, Vorkosigan Adventures, and older editions include G: Humanx Commonwealth (Based upon the Flinx adventures), G: Lensman, G: Uplift, G:Terradyne, G:Tales of the Solar Patrol, and G: Alpha Centauri... and that's just the space opera stuff. Most of which is before 2013...
G:Space has a much more realistic system generation.
G:Space has a more realistic ship system
G:Humanx has a much better space opera combat system and a much more consistent ship system.
G:Lensman likewise has a different combat system.
Further, the only reason for SJG to have paid Marc for rights was for the setting, since GT used no actual Traveller mechanics outside worlds...
So, except for a few Old Guard types, and the Mongoose and CE recent additions, Traveller has always been OTU centric. (Unless you count T:2300 as Traveller...)
Note also: T5 is every bit as grounded in the OTU as MT, TNE, or T4 were/are. The OTU is in Marc's head, and it has become a major part of his conception of Traveller.