"Spitting out an mp4 file that you can copy to another device" is not a service that YouTube provides. That's why youtube-dl exists. If it was a service that YouTube already provided then there would not be as much reason for youtube-dl to exist.

"Unenforceable" doesn't mean practically unenforceable, it means legally unenforceable. The laws of physics permit most cars to exceed most speed limits. Those laws are still enforceable.

Unenforceable clauses in contracts are things that are either contrary to law or otherwise don't fall under contract law. So a clause that says "if you sign this you must give up your firstborn son" is unenforceable. That sentence just says that "if we screwed up drafting this contract, just because you were able to challenge one clause due to being unenforceable doesn't mean the whole contract is void." It's a severability clause.