BC should be given, by all accounts. And while it hasn't been officially announced yet, I'm completely certain Horizon 2 has been in development since Guerrilla released Frozen Wilds (since the release of the first game even WRT concepting and story, after it became apparent the game is a hit). The development of Horizon took very long, but much of that time was spent on the engine/tools - Guerrilla has stated that they wish to shorten their development times, and in the state Decima is currently in, I think it's very much realistic (that
GPU-based procedural placement system is just amazing - many of the environments in Horizon are just the result of designers "painting" different types of vegetation and such on a terrain, with no manual adjusting afterwards).
It also doesn't hurt with the development times that they're soon locating to a new office with their own in-house mocap studio and hiring over 100 new employees. And it's not like making a launch title would be foreign for them - while apparently it isn't that great as a game (haven't played it myself), by all accounts Shadow Fall was technologically a very impressive launch title. So Horizon 2, PS5 launch title (or launching pretty close to the console), I'd find it quite likely even.
The only point I don't agree with you is the year - and the more I think about it, the more it makes sense, hear me out. I'm partially in camp 2020 because of mass-availability/-affordability of 7nm AMD APUs, but let's ignore the tech. Sony isn't organising PSX this year, because (paraphrasing) they don't have enough to wow the audience with. And that may be true, this generation is starting to wind down, there will likely be no major exclusives for this generation anymore. PS4 was announced in a press conference, which they could do this time around as well, but.... boooooring. The news will spread quickly, sure, but that's not how you get the hype machine going. E3, eh, better - but PSX 2019, it would be so perfect. Most of the same journalists will be there as would at E3, but more importantly, the place is filled to the brim with PS-fans, many of whom surely are also vocal/active online. They don't even need to have a playable unit on display - just show briefly what the latest black plastic box looks like and the fans will do their best to set the Internet on fire. That's the kind of grassroots visibility/marketing that many companies can only dream of, and you can't really buy it, you have to be clever about it.
As said, I don't quite agree myself, but just as a ballpark figure.
