“Correct” is a strong word. It isn’t defined in https://tools.ietf.org/html/rfc4180 so Excel should not try to be smart and add extensions only they support.
As for the actual RFC, it's worth taking a read. Any sort of value interpretation is left up to the implementation, to the extent that Excel's behavior in interpreting formulae is 100% in compliance with the spec.
Anyway the RFC doesn't mandate any value interpretation IIRC.