Quote Originally Posted by Greywander View Post
If the Earth was round, then people living in the Southern Hemisphere would fall off. You can't take a demonstrably false example and use it to prove something more ambiguous. It is specifically called out in the rules that a multiclassed character can have 9th level slots but not be able to learn any 9th level spells. This has nothing to do with whether the level requirements for an invocation refer to your character level or warlock level, as until the clarification from the errata, there were no clear rules about this.

Reductio ad absurdum would require you to find a similar case with similar ambiguity where the ability breaks down if it is allowed to scale with character level rather than class level. The Spellcasting feature would be a good example of this, if the ambiguity wasn't clarified elsewhere in the rules already.

We already have an example of the opposite with cantrips, which do scale with character level, not class level. This means we have a precedent for abilities to scale with character level rather than class level. Not every ability will work this way, but what it means is that some of them could, and do.
Cantrips only scale with character level because the rules on pg 164 say so.

There is nothing on pages 163 and 164 that says Warlock Invocation prerequisites are character level, so they aren't.

I will repeat: The rules for multiclassing are on pages 163 and 164. They are variant rules. The rest of the rules are not written to refer to the multiclassing rules. They are not part of the base game.