From what Snape had heard, the Tranquility was alleged to be so large a ship as to be massive. So much so that the odds of never seeing the same person twice must be fairly high. It would appear that he's not going to be so lucky in this respect.

"Don't let me stop you." Which could be interpreted as an affable, if not practically jovial response... coming from anyone else. The lip-curl might be a hint that such was not the case here.