It’s describing a group of related languages using their common ancestry.
To use the example from the video, it’s more like calling French, Spanish and Italian “Romanish” languages. Or, you know, “Romance” languages. In that case, they both sprouted from the language of the first elves as various groups of said people separated and evolved their own linguistic style from the language their ancestors originally spoke (haven’t read the whole Silmarillion but I’m pretty sure that’s the idea)
