Devs are making these games on PCs. As an actual developer from Moon Studios said in one of the Resetera threads on this issue, the specs are pretty much known. Whether you have a dev kit this early doesn’t matter. Final dev kits for both consoles aren’t coming until 2020.
Agreed, let's be honest it's not going to be much different from last gen. The systems Sony and MS put out are going to be very similar.
Both have Zen 2 CPU's, both have NAVI GPU's, both have SSD's, both have some form of Ray Tracing support. Not having the Scarlett dev kit isn't going to be much of a hurdle. As long as they get it in time to make adjustments and optimisations for the few system specific quirks. (I.E things like the XBO's ESRAM). Even if they didn't get it till May I don't see how the games would suffer much.
LTD: PS4 - 125m, Switch - 110m, XBO - 51m
2020: PS4 - 9m, Switch - 22.5m, XBO - 2.5m, PS5 - 4.5m, XBX - 2.8m