It seems obvious that the SFU can't be fully detailed in one book, just as the OTU couldn't be. Even C-Stars is a two book deal (for Traveller) and it's just one solar system with a lot of information that's going to be left out and expanded upon using The Void rules.
Except that the OTU Core book only mentions the "we will have this in a future supplement" about small craft construction rules. Unless you wanted custom small craft straight away, you play with the Trav Core only, as I have done for ages until I actually picked up High Guard.
I agree you need High Guard along with Cstars to play that
supplement.
What I meant previously was, although I realise you'd probably need a Trav Core to play PD: Trav, I'd be annoyed if rules are introduced which feature heavily in the game, then they wont be explained until a future supplement arrives - the small craft construction in Traveller wasnt a huge part of the game, but things like omitting entire bits (if not using the Traveller equivalents), such as entire spaceship rules, equipment lists, lists of planets, naming a skill but not saying what it does until a future book, stupid things like that, thats an annoyance.
Now I am looking forward to seeing what PD: Traveller is like, so if its a simple case of, with the Klingon example, that there are base profiles and to learn more I have to get that sourcebook, fine thats cool, because I can still use them in my games until the book comes out. Also theres Star Trek wikis anyway

Its more the case of saying there are Klingons as a main combatant and not having
anything about them after that in the Core until the supplement, making me wait until that supplement book is out to use them, thats what I would find annoying.
I know rpg books is a business, but that whole "luring someone with a note on a string and pulling it whenever they get near" makes me
not want to buy more. If the bare bones are there and I need more info, then I am more inclined to buy the supplement when its released.