Well, yes, nevertheless it is a contradiction: either there is an expectation that others treat it as confidential, then no explicit statement should be needed, or it is not, then it should be needed. As it maybe unclear, what the expectation is, at least a rule (!) should show unambiguously what the legit expectation is.
So these are not rules (indeed, it is called "guidelines"), but recommendations ... I would prefer something that is less ambiguous and thus closer to a rule, but of course I see that these guidelines just try to be practicable (and not normative ...) ...
I think there is a slight contradiction between paragraph 10 and 11. If a PM should always be regarded as confidential (11), there should be no need to mark it as such explicitely (10). But I think this is just a tribute to the difference between an idealistic approach and reality, so I am fine with that ... Maybe I would just reorder 10 and 11 - 11, which contains the rule (expectation), should be first, 10, which contains the realistic warning, should follow ...
I read that as assuming that you should always treat anything someone sends you as confidential, while not making the same assumption about what you send to others unless explicitly stated as such.