It wasn't considered as gendered, as referring to humanity as "man" is a holdover from when "man" wasn't ever gendered; we don't have any recordings of it specifically referring to males until around 1000 CE.

The old words for male/female were "wer" (see: werewolf) and "wīf", the latter of which diverged into "wifmann" ("female human"), later "woman", and "wife", specifically referring to a married woman. You still see "wife" used without implication of marriage status in words like "midwife".

Anyway tl;dr "man" historically wasn't gendered, hence it commonly being used to refer to humanity as a whole even in modern use. Also it more accurately states that no humans have been there before, rather than discounting present natives.

Edit: also, as another comment played on, this was used as wordplay in the Lord of the Rings, in which humanity is referred to as "the race of man", where a prophecy refers to no man being able to defeat one of the antagonists but doesn't specify that a woman can't.