There are oodles of entities who have trademarked the term 'Traveller' - two Ls -
search yourself and see. What you're saying is that Mongoose is in danger unless they change the name of the game,
and that is wrong by black letter law.
No entity can bring
trademark action against another entity who is using a term they have trademarked
outside the very narrow class of goods their trademark is granted for. Period.
I'm aware of the universal prevalence of the word "Traveller" - try reading Bluesky feeds about the game and you'll learn a lot about a certain ethnic minority but only see a post now and again about the scifi RPG. I am also NOT suggesting Mongoose change the name of the game - that would be exactly the wrong thing to do.
WotC knew exactly what they were doing when they wrote Traveler's Handbook on their book; that did not happen by accident, and the reason has to do with the existence and value of the Traveller RPG. Their lawyers would have looked at it, and decided that Mongoose wouldn't easily win a legal action against them. This is because, as you say, trademarks are interpreted narrowly.
The real question is why did WotC decide to do this? It makes them look pretty feeble, like a cheap knock-off brand that tries to fool people into thinking its the real thing (which is kind of what this is). It could be just this, and it won't go any further unless their game takes off, which it shouldn't do, but who knows?
They might also be trying to move in on the IP for later action. Traveller has a lot of IP managed in a not very professional way, precisely because it has been open to a Creative Commons kind of approach. No matter if you get contracts looked at by a lawyer, hobbyists and small scale entrepreneurs just won't be able to afford the expertise and consistent strategy of a corporate legal department, so there will be a lot of scope to use legal action to capture IP. If WotC decide it is time to move in and start monetizing Traveller, then they'll buy or steal what they can, sue anybody else still left publishing Traveller branded products, and try to bring the whole thing into their IP, so that anything happening in the space involves money flowing in their direction. Part of this would be to establish some kind of legitimacy - even if fake legitimacy - by having some products of their own that they can portray as somehow being a part of the Traveller scene.
If anybody brings a trademark action, it would be WotC after they've established themselves in the Traveller market, and they would do it not by trying to win a case - which wouldn't happen, because of the aforementioned narrow character of trademarks - but by establishing a plausible enough case to take it to court, preferably in a carefully selected jurisdiction which is favourable to them and expensive for Mongoose (or whoever is being sued) to fight the case in. If it doesn't get thrown out straight away, since WotC has deep pockets they can keep legal actions going forever, and this would force Mongoose to surrender. That's the normal way this is done.
This doesn't mean anything will come of this. Maybe they're just trying it out and will decide not to go this way, for whatever reason. Maybe their game will succeed on its own merits and they'll be happy to compete fair and square with the real Traveller.
