The definition of errata has nothing to do with official play; it’s not a term invented by WotC.
The dictionary defines it as “a list of corrected errors appended to a book or published in a subsequent issue of a journal”.
I’d agree that this doesn’t meet that definition. It’s an addition, not a change.
Correcting an error that changes a rule does make that old ruling unusable in official play. That was my point.