I think anyone who turned up in full knowledge of the boycott is of questionable morals / judgment, but as I've said on another thread not everyone knows about these things, regardless of how well they are communicated. It's beyond debate that some people there on Tuesday won't have known about the boycott, the amount could have been 1, 10, 100, 1000 but there will have been a number amidst the crowd in that camp. As others have said, the boycott was a raging success given total boycott wasn't ever really realistic.
In terms of the atmosphere, Tuesday night was probably akin to what it would have been if there were 30,000 of me crammed into the home stands. I'm as introvert as they come. I turn up and I clap in all the right places and cheer goals, but that's all I do, because that's my personality type. I display the conduct of a snooker fan attending a football match but that doesn't make me any less of a supporter than someone who chants for 90+ minutes, I just internalise things that other people externalise. I think there's probably a decent overlap in terms of introverts and people who don't get to hear about things like boycotts, so it would stack up that the crowd was comprised of more introverts than extroverts which led to a quiet atmosphere.
Obviously all just opinion and conjecture but that's my thoughts on it.