It would also be pretty ironic if so, given that the idea was basically lifted from Google internal infra.

Don't confuse a trademark with a patent. Even if Google uses "go links" internally, the fact remains that the trademark is registered and in use by another entity.

I mean, TinyURL launched in 2002.

Point taken, but I think it's a different concept -- named internal short-links versus randomly generated small strings for longer links.

I was going to say the terminology comes from inside Google, where afaik go links were invented. However, yeah, that's straight up a trademark. You'd need a lawyer to know whether the trademark would survive litigation, and whether the s on those trademarks makes a material difference. (OP isn't plural, those trademarks are)

How can they defend a trademark that people used (at Google) before them?

A trademark is not a patent, you don't need to prove you were first to have "the idea". [IANAL] It's the first to use the term in a commercial context, which Google has not.

Trademark was found via a Hackernews thread? You guys are good :) We have three trademarks that we've defended successfully before. Would encourage anyone wanting to try to use go/links, to just sign up to https://www.golinks.io ... it's completely free to use, forever and you can get started in a few seconds. We're trying to get go/links everywhere and are happy to be at the forefront of that :)