Well, 6.b3?! does weaken the long diaganol for no good reason.  In these sorts of opposing fianchetto positions, the queenside one is nearly always weaker, because it's unprotected and susceptible to tactics on the long diaganol.  Think about the d4-d5 thrust in the Queen's Indian, meeting ...exd5 with Nf3-h4, for instance.

Still, it's hard to believe that White would actually be worse.

yesterday I lost a game due to a bad opening as white ... I mean openingwise I can play almost everything by feeling, any opening ... but originally I wanted to play 1.e4, but then I saw my opponent plays the french and I am not feeling so fit right now ...
and I wanted to play the king's indian attack as white ...

so 1.Nf3 d5 2.g3 Nf6 3.Bg2 g6 4.0-0 Bg7,
somehow I decided now not to play d3 as in the KIA, but went for d4, d3 just didn't seem appropriate against a fianchetto setup ...  actually right now we are in a gruenfeld ...
so 5. d4 0-0 .... notice that we right now have a symmetrical position .... and now I played 6. b3?? c5 and I am already worse!!! One move from a symmetrical position one tempo up and you are already worse!!! Unbelieveable!
I mean b3 is often played vs the king's indian or the dutch ... so I thought I just play b3 here ... but I thought so wrong, actually I am in a Gruenfeld, and there is a reason I never saw b3 being played in the Gruenfeld!!! Bethanks to this game I really know why ... the engine like stockfish give only -0.2 or something, but in reality you are even much worse ... you are not anymore white, you are black now .... your whole way how you build up your pieces everything is strained ... the game of black is played by itself with easy to find natural moves while you see your own position fold and you can't do anything about it ... it's so crazy, and after the game the opponent said he would have played the scandinavian ... anyway I should just have played 1.e4 or should have went for d3 as in the KIA ... this purely positional chess is just not my cup of tea!