As is usual with communication, the answer of "who creates the resulting effect" is "both the speaker and listener, not one or the other". Particularly, in this case, the speaker's contribution comes by choosing a term which has both a male-gender-specific and a gender-neutral sense rather than something unambiguously gender-neutral.

Are you defining ambiguity by the number of people who report to have each interpretation? I suspect if you asked 1,000 American native English speakers (I don't know about English in other countries) how they interpreted that usage of "guys," the overwhelming majority would respond with the gender-neutral interpretation. Even among the people who are offended by the potential male-only interpretation, I suspect that for most it would be clear that they actually did understand the speaker's intentions.