It’d be more accurate to say that a singular “guy” is undeniably masculine, but “guys” doesn’t assume that everyone in the group is a guy. Like, a group of 1 man and 9 women could be referred to as “guys” and it’d still be considered proper, but remove that one guy and leave only the women and suddenly “guys” becomes somewhat improper, unless you’re one of those people who uses “guys” regardless of the gender distribution of a group (which is not that uncommon these days).
It’d be more accurate to say that a singular “guy” is undeniably masculine, but “guys” doesn’t assume that everyone in the group is a guy. Like, a group of 1 man and 9 women could be referred to as “guys” and it’d still be considered proper, but remove that one guy and leave only the women and suddenly “guys” becomes somewhat improper, unless you’re one of those people who uses “guys” regardless of the gender distribution of a group (which is not that uncommon these days).