One of the people summoned just rolled with it and another beat him up. That might be a good enough explanation.

When Childermass starts talking, Strange turns to look at him...and instantly counters that look with a slightly defensive one of his own. He turns back to look at the basin, hoping that something'll happen, just to get rid of this feeling he's got like a schoolboy being chastised by his teacher (which is a stupid feeling to have in the first place, this is Childermass, not Norrell). Unfortunately, nothing's really happening for him to divert the conversation.

So at the question, Strange looks up from the basin and gives Childermass a look that's slightly confused as well as slightly amused. At least to him, the answer is obvious. "I tried simply to see if I could do it here."

This shouldn't come as a surprise. Strange has always been the one to say that magic needed to be explored, boundaries needed to be pushed, what was the use of the unexplained if not to explain it? And, apparently, testing the limitations on summoning faeries counted as exploring.