This is mostly about social signalling. The presence of the bot conspicuously telling people not to use the term "guys" signals to both men and women on the tea

This is mostly about social signalling. The presence of the bot conspicuously telling people not to use the term "guys" signals to both men and women on the team that influential people on the team are feminist. So what matters isn't whether or not "guys" is technically a valid term for a mixed-gender group; what matters is that the ban on "guys" will be interpreted as feminist.