The way Sol sees it - already, because of course - what he's getting here is a concession and he's willing to respect that, and now that Bruce has made it, he's not going to demand more than he's willing to give. He lets aside the question unanswered because it isn't like it's hard to find out in a lot of ways that he has a child; for one thing, he could easily have just been seen dropping her off. The importance of the distinction makes sense with the explanation he does get, and-- it doesn't bother him, not immediately:

“I was probably married, then,” he says, thoughtfully, twitching his cigarette absently between his fingers. (The stress-tension in him is not necessarily unfamiliar; he's working through something, but he's doing it quietly and in his own time, respectful of his own feelings as of anyone else's, even when his feelings are 'annoyed at having feelings'.) “I adopted Liesl as a baby, after my wife left.”

(How he thinks of their divorce and how he describes it - and it was a mutual decision - often depends on how he's feeling about his family in general.)