This makes sense to me. I do think it's a new proper faction because there were warscroll cards, but the models could act like a mercenary faction and therefore the tome includes all the different rules for other mixing with other factions. Maybe? Dunno on that one. Seems most likely that more units surely have to be coming, as you said.
Then again, GW are GW and they're not exactly against releasing small armies with limited kits. Flesh Eater Courts are an example. There are a total of 4 kits? Can't quite remember. Not to mention Archaon and the Varanguard are considered a viable way to play by GW. So maybe they view each gargant model as having their own rules, plus a multipart new hero that is massive like Archaon, terrain and endless spells, and they make up the lack of extra units by having some allying/mercenary capacity?
I'm not saying I think that's a good idea. Just that seeing the Warscroll Cards shipped this far in advance makes me think it's somewhere in the nearish future rather than ages away, and I can't see them doing a proper expanded new army so close to Lumineth. Just a gut feeling, I suppose.