I remember watching some frag vids made by NA players (that later ended up playing on 6v6 ESEA Invite) and they all featured clips from 7v7 or 8v8, i even saw 2 medics, and they played some weird custom maps along with maps as cp_well, yukon etc (those last two have been played till 2010-11 i think)
And as other users explained, competitive team fortress was a thing before tf2 (with tfc and qwtf) so it was bound to happen with this one aswell