I let Houdini look at the position after 1.b3 for a little more than five minutes. According to Houdini, White has no advantage at all, let alone "a clear advantage."
28 [+0.00] 1.... f6 2.h3 b5 3.Ne3 c5 4.Bh2 Nc6 5.O-O Bf7 6.Nc3 Nd4 7.f3 Qc6 8.Ne4 Ng6 9.c3 Ne6 10.a4 a6 11.c4 bxc4 12.bxc4 O-O 13.Nd5 Nc7 14.Ne3 Ne6 15.Nd5 (351.62)
Apparently Black's position is not compromised after 1...f6 (or White can't take advantage of it) and Black's d8 bishop will find activity via the a4-d8 diagonal.
I was looking into chess960 starting positions and came across QNRBKNBR After 1.b3 doesn't white already have a clear advantage? black is basically forced between 1...Ne6?! which must be bad since it blocks the e pawn and makes developing both black bishops very difficult. 1...f6 which is probably best but still weakens blacks king side and takes away a diagonal from the DSB which may have been quite useful. There might also be 1...e5 intending 2.Qxe5+ Ne6 followed by attacking the queen for rapid development and ...b6 somewhere to cause white some problems. This gambit is most likely unsound but is worth looking into because it might be playable between two club strength players in a boots game. What do people think? isn't the starting position unfair since it favors white so much?