Mark Laidlaw doesn't know what the UU is but he's super bad at lore and admits the community has developed layers of meaning around it- I don't see Mark Laidlaw seething at his creation being ruined looks to me like he's accepted the community has given a definition to the Universal Union
Tbh. UU Defenders' emphasize of Marc Laidlaw supposedly cheering GMOD Roleplay community for renaming the Combine to the Universial Union is a little overexaggerated. He doesn't think the fanmade term to be "baffling" or a glorious substitute name for the Combine, it's just more like he was like "Oh... Huh, I guess that was a thing by roleplayers at one point" and moved on to never really dwell on it again.
In my personal opinion on the matter, I just think the Combine or Breen using misleading labels for themselves such as "The Universal Union" is counter-productive to their bloodstained principles of extreme rationalization and anti-idolization, and just to as of who they really are in general. The Combine is more like an undefinable "Embodiment" or "Promise" with double meaning; like the Combine Harvesters you would see in today's farms, or a system to transcend their consciousness between life and death (
Half-Life)
I don't tend to observe the big baddies behind the Combine as textbook "conquerors/imperials/rulers", but just godly space worms who experienced an existential crisis (such as the fear nothingness after death) and are actively medicating themselves to not die of natural causes. They certainly don't have any interest governing Earth, aside from harvesting the planets for its resources and to cull humanity for transhumization (Stalkers, Soldiers, Advisors). The Shu'ulathoi already have their enormous Overworld they're also modifying to survive past the Big Freeze-- Hell they only saw the Gman as a DNA Sequence and tried using him to create a literal
bacteriophage (a known viral structure used to kill Bacteria). Conquering other planets for prolonged occupation would be useless.
My beliefs aside... Marc Laidlaw had actually made past statements that he doesn't believe in canon when making HL2, so trying to make a server "lore accurate" is utterly useless. Everyone's free to call the Combine to in whatever way they want. We can probably just kill two birds with one stone and just name the bill that Breen signed to "The Universal Union Act".