There is no semantic significance. The syntax is in fact ambiguous insofar as it may be impossible to tell in which of several possible ways a given comment or WS was produced. However, this does not lead to semantic ambiguity because, unless specifically stated otherwise, the presence of absence of a comment or additional WS has no semantic meaning and, in particular, it is a matter of indifference whether it forms a part of the syntactic construct preceding it or the one following it. The automaton should swallow up all CFWS wherever it sees it and just take note that the preceding non-CFWS item has been terminated. This is what RFC 822 parsers have been doing for years.
Comments identify the agent.
That wording seems to me to imply that products and comments are treated on an equal footing. Either may be used to identify an agent or subproduct, but there is no suggestion that a comment is to be specifically associated with either the thing following it or the thing preceding it, so far as I can see. OTOH there seems a definite intention, in section 4.2, to follow RFC822 practice. But I see no such codification, apart from the sentence you have already quoted. OK, that can be debated.