Newbie question - Does this mean you're just going to add seed to an
existing lawn?

Yes. I have two acres. The front and sides are great, but the back
is very sparse. My fault for not dethatching before the pre-emergent
was put down.

Because of how huge my backyard is, it's not feasible for me to put
topsoil down over the seed, so I'm still looking into the best way to
overseed.

Hopefully someone with more knowledgable will chime in here, but I'm
not sure if the seed germinates in the fall, or if it lays dormant
until spring.