LÖVE

My sibling was trying to get a handle on networking, and he followed this tutorial:
https://love2d.org/wiki/Tutorial:Networking_with_UDP

But apparently, the example source code they provided doesn't even work.

Who on Earth do you contact for that? Is it normal to just have a bunch of hit-and-miss tutorials on the wiki? Is there no basic moderation?
This is a bit disappointing, because the LOVE Wiki is a great "bible" of sorts for us love users - and I'd like to know it can be trusted.

Just wondering if this is normal. (And perhaps if anyone has a better UDP networking tutorial)

Depends on your networking setup/configuration. You can't blame the wiki, it's more about understanding what's going on.
I did write a similar tutorial:
https://2dengine.com/?p=networking
It's definitely possible to get it working over LAN but running it over the internet is more complicated.

Bypassing NATs over UDP will require hole punching. IPv6 wouldn't have NATs (and this problem by extension) but it still hadn't caught on fully. Beyond that, relevant ports must be open in the firewall.

Networking is right about the most difficult thing to implement in a video game. Expect fully this to be excruciating.

Wiki editing is somewhat a communal project in that the community, i.e. forum users, can also edit with their forum credentials; the wiki also notates version differences on most reference articles. Simply put, no one's getting paid nor has the drive (apparently) to make unified examples for all articles, and make them version-dependent also.