My somewhat facetious answer is that wanker is British slang for someone who masturbates, so given that all of U2 are males, I assume all have at some point indulged in self-love and perhaps still do. I'm told even females sometimes indulge themselves also.
My more serious answer is - hmm, tbh, I was listening to a POP era concert recently, and during Bullet the Blue Sky Bono makes a passing reference to the Iraqis being bombed - so given the timescale I assume it was around the time Mr Clinton ordered a bombing of Iraq targets - due, some claimed, because Mr Clinton wanted to distract attention from his then marital woes.
My point is, Bono was still prepared to criticise even those politicians that he liked circa late 1990s.
These days, obviously, he's not prepared to do that - he's Davos man. So I would argue that sometime in either in the late 90s or early 00s Bono became a wanker. Possibly, the 911 attacks were a game changer for him personally - but ATCYLB, released of course almost a year before was a boring, dull-as-dishwater album which caused no offence to anyone, politicians or others, so who knows.