When I was growing up in Iowa in the 1930’s you used the pot only for uninating. Otherwise you stumbled out into the dark, even in the middle of February, to use the outhouse. Barring “the runs,” it is usually possible to wait until morning anyway.
And outhouses didn’t only last until the early 1900’s. Most farms had them up until post WWII. Indoor plumbing didn’t come to my part of Iowa until well after rural electrification, which came just prior to WWII, when you could have a motor driven pump to provide water.
It’s rather more likely that he was alluding to drunken students using them for vomitting. This would have been one sound which, although not unknown elsewhere, would have been much more common in a Cambridge* college.
Russell was a student and don at Cambridge, not Oxford.