It would seem strange that Apple would release two case modifications to the same machine within a year of each other. A new keyboard is going to require at least some degree of modification to the case tooling. It could be very modest if the new keyboard is only slightly thicker than the old, or it could be significant if it was a very different keyboard technology.
If we were a couple of years from the release of a redesigned MacBook Pro, I could easily see Apple releasing a machine with a slight case retooling, to get away from the butterfly keyboard as quickly as possible - but we are less than a year from when we would expect a regularly scheduled redesign. It makes far more sense to push the redesign (16" screen, possibly other modifications - cooling, etc.) forward by those few months, in order to release the keyboard when it's ready than it does to release two machines that require expensive retooling months apart. The third option would be to hold the keyboard back, and release everything together in 2020.
Ordinarily, I'd think a spring 2020 release the most likely - but there are a couple of things that lead me to believe it's probably actually coming this fall. Most important is all the pundits who seem to have sources in this direction. Second is the next 45w processor Intel's going to release - it's not 10nm Ice Lake, but 14nm Comet Lake (a minor upgrade to the existing chips). There may even be another 14nm++++++++++ step stuck in there after Comet Lake - Rocket Lake, which will be coming to a desktop near you, and we aren't sure about 45w mobile. This places 10nm 45w mobile in the "still a long way away" category - spring 2021 is the soonest we might expect it (although there is an off chance that Comet Lake might appear this fall, followed by a 10nm 45w chip in fall 2020).
The third piece of evidence that Apple knows, but the public doesn't, is Intel's and AMD's expected delivery dates for CPUs and GPUs, respectively. If Apple knows that mobile Navi is either very close (and will make it into a Fall 2019 Mac) or far enough away that a Spring 2020 Mac won't have it, that could encourage them to go ahead and release this fall. Similarly, Apple knows the specs and release date of Comet Lake-H . If they know it's very close, they could know that a fall release could include it. If they know it's a minor update, they would know not to hold up the keyboard, the screen and possibly Navi waiting for it. It's probably not a major update coming in the spiring, because that's the one thing that would cause them to wait. It doesn't matter that we uninformed civilians don't have access to that info, Apple does - and they are acting on it.