I'm not sure why you would expect big-ish announcements in a Direct but not at Sony's TGS tbh. Outside of maybe Bokujo Monogatari(sp?) any new, notable game for the Switch is going to be multiplatform, and for those games the PS4 will be the lead platform(both development and promotion wise) so they're probably more likely to be announced in a Sony event than a Nintendo one.
Nirolak made a good point about the PS4 and nobody has posted anything about expecting big third party announcements for the Switch in the Nintendo Direct. I don't know where you are getting that from.
Also, the bold probably isn't a safe assumption lol.