This sounds odd to me, unless you connected to an Ethernet port behind a desk or somehow forced open a network closet… They also might not like it if you disconnected one of the public computers to use its cable/port; otherwise if this was an open and public port, you used it as designed and the librarian probably has watched too many Hollywood hacking movies. I have to admit, I never thought of this as a way to bypass the captive portal (sorta just assumed everyone going through the public network would have to hit it, kinda of the equivalent to having everyone sign a liability waiver).

With that said, I can see some institutions not liking connections that aren’t part of the more traditional/commercial networking (but it doesn’t sound like the library took issue with your traffic, just the librarian didn’t PHY link you chose to use). For the SMS thing (I haven’t seen that used in a while, you might be able to use some sort of burner number app if they don’t filter them).