Not silly. It's a clarifying question. It's valid.
Does this not just throw chaff across the radar screen as far as "which version of Traveller should I play?"
CT and MT are there for Old School Travellers.
TNE and T4 are there for historical completeness. (Sorry to the 5 people who may be offended at that remark, I'm not an official spokesperson so just ignore my hasty generalizations.)
GURPS:Traveller, while having a current license, is somewhat of a grognard item, as well.
And there is Traveller5, a reference work laying down the rulesets of a dozen smaller books (equivalent to LBB1 thru LBB8 and more), undergoing its erratastorm. While accessible to Old School Travellers, it is not accessible to people not fluent in Traveller. For that we need the errata-fixed text, and then a distilled Player's Guide, etc.
That leaves Mongoose Traveller, which is the current Traveller ruleset for new Traveller gamers, warts and all. And there are warts, but no more than any other ruleset I suppose.
Liftoff is intended to be the "Starter Traveller" for Mongoose Traveller. At this draft stage the rules are draft, and as Wil has mentioned, it's
not yet Mongoose Traveller rules. That will change.