In the examples you mention - you're establishing that there is an Imperial Interstellar Navy (and an Empire, for that matter), that there are barbarians who prefer greataxes and eschew armor, and that there are armored knights. None of those are necessarily givens in any particular setting. Star Trek, for example, doesn't have an Imperial Interstellar Navy - at least not on the Federation side - so you're clearly not doing a far future utopian/Star Trek campaign. A campaign based around the Three Musketeers is unlikely to have greataxe-wielding barbarians or armored knights (in the traditional Medieval Romance or Game of Thrones mode anyway) so you're probably not doing a campaign like that. By establishing the parameters of the setting, you are worldbuilding.
I think I have two reasons for disagreeing with this.Ok again maybe I'm not grasping this but in establishing armoured knights or barbarians with great axes we are in fact establishing part of the world...that these things exist in said world. How is that not worldbuilding? Likewise deciding that kobolds are mini-dragons who serve dragons dictates the culture of kobolds (as a whole) in your particular world. I get you don't consider it worldbuilding but I feel like you haven't given a coherent reason as to why this doesn't count. I also think it might help if you provide some examples of what you do consider worldbuilding to contrast.
One is about the relationship between commercial products and actual play. I bought the 4e Monster Manual. I use it as my default source of lore for my main 4e game - I told the players that at the start of the campaign (ie "I want to run a default 4e game - who's in?"), and have stuck to it. The MM tells me that orcs worship Gruumsh, so that's the default in my game.
But there has not been a single occurence of an orc in that campaign. There have been goblins, and hobgoblins, and bugbears; ogres and various sorts of giants; I think some troglodytes; but no orcs (and no kobolds, lizardfolk, xvarts, and probably other fairly common humanoids I'm forgetting). So does the world in fact contain Gruumsh-worshipping orcs at all? Who knows? A disposition to say "yes" should it ever come up, because that's what the book says, isn't the same as it actually having come up in play.
(The same reasoning applies to PC build elements. Do wardens exist in the world of my 4e game? Who knows? No player has ever built one, and I've never used a warden-type character as a NPC.)
The second reason follows on from the first: as far as RPGing is concerned, until some concrete situation is established in which some PCs are present, the game isn't happening. And worldbuilding in the RPG context therefore has to be in service of that. So long as it stays at the abstract level ("Kobolds serve dragons") then no setting for play has been established. It's just the GM daydreaming to him-/herself. (Maybe the daydream gets written down in a notebook. It's still just a daydream.)
Once something gets written down about a dragon being here, or having done this thing in the past, with kobolds being involved in this way - now we have setting that can feed into situation, which is how I would think about RPG worldbuilding.