This is against not in favour of your argument - it's a case of Bitly enabling users to still use other systems for tracking (UTM) while using a link shortener (as said by person you replied to), not evidence of tracking being the purpose of the link shortener.

Bitly does have their own basic analytics built in, so some people do use it for tracking, but that's still a secondary function for people who don't care enough to set up independent tracking. It's not a sale pitch of "if tracking is what you want, link shortening is the solution".

And the t.co example isn't actually useful for spam prevention, if Twitter decides that a certain URL is considered spam they can just as easily remove that link from any tweets trying to use it as they can disable a t.co. Meanwhile the t.co does add friction for users seeing whether the destination URL looks likely to be spam. I don't think I've ever heard anyone before argue that link shorteners are a good thing for fighting spam, only either bad or at best neutral.