So "'s" is what's called a "clitic". It's a tiny little piece of meaning that can't stand by itself and has to "lean" on a neighboring thing to be grammatical.

The interesting thing is that their distribution is syntactic, not morphological. So, instead of attaching to a word, like affixes do, "'s" instead attaches to entire noun phrases. This includes all adjectives, prepositional phrases, and even subordinate clauses, as long as they're part of the possessor noun phrase.

So, "the dude's car"? Perfectly fine, and it even looks like an affix here. "The dude over there's car"? Perfectly fine. "The dude I went to school with but who forgot that he ate a capybara yesterday's car"? Perfectly grammatical in English thanks to the power of clitics.

Bonus fun fact: "'s" used to actually be a suffix, but somehow became separated over time, and it's a big deal in diachronic syntactic theory, because things are only ever supposed to evolve toward being a suffix, but "'s" is one of the few things that seems like it evolved the other way, which throws a wrench into how we usually view the process (called "grammaticalization").

In short, Anon's sentence is a perfectly cromulent use of the English language.