It's not required but they need to understand the implications. In this case the implication is that Google drops the mail. So clearly they didn't understand the implications.
Yes, but this might be googles fault for not respecting/missinterpretating the spec.
Google probably did parse these messages as well-formed before inspecting them and deciding to drop them based on the lack of this field. The RFC imposes no mandatory obligation to deliver messages just because they are well-formed.
So you agree that calling it a requirement is wrong?