Exactly.
The DMG does specify caster level for arcane scrolls when it is important, because (for example) there's a big difference between a magic missile cast at first level versus one at seventh level.
OTOH, the DMG does not specify "cleric-created CSW" or "4th-level CSW." So, inferentially, it must be the caster's level, and the not scroll scrivener's level, that controls.
As for "more bonus languages than me" implying that Elan must have at least one bonus language, any bonus language is greater than having none. While it would be more precise if Elan had said, "I know you have bonus languages and I do not...", I think it is just an economy-of-words issue. The joke is funnier the way it is written, compared to a verbiager-but-more-precise alternative.