So 'mostly' is the clearest they get here. Meaning more than 50% of lines of code? At any given time?

This makes sense from a copyright perspective -- as long as you maintain 50% or more actual human authorship, in case of a conflict with future copyright laws around LLM generated code, you can at least claim that a majority of the code is human-authored.

They are unable to be more clear, because the law is unclear. The safe assumption would be "if the code would be copyrightable, had it been written by a human, then it must have been written by a human". It is impossible to be more clear than this, because "copyrightable" is a legal term of art decided by judges.