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 like the 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).
The concern for code duplication is valid, but as the article mentioned it is also a while off until the Nova project is mainlined. I honestly never thought of how the work to bring in Rust to mainline may in effect lead to a more complete deprecation of older hardware as we start to change API’s older/unmaintained components aren’t updated. On the flip side, trimming out older stuff might save maintainer work going forward.