Also depends what "in-depth" means, as such.
Which, now that I think about it, I'm wondering what material, outside of what we know, will be in each book? And, more importantly, how many pages will each book be? I'd have to assume they must add up to more than Eberron's 320 pages, or they'd print them as one big book.
One point of comparison we can make now is that the Baldur's Gate guide in
Descent to Avernus was 50 pages long. Assuming similar sizes for all 5 mentioned areas (and they may just reprint or slightly modify that same Baldur's Gate guide in this upcoming book, as it's pretty decent, and would save them a lot of time and effort, although no doubt there would be some outcry by some quarters if they did), that would be around 250 pages. If some are a bit shorter, we'd have room for a bestiary and make it 256 pages, which is one of the most common 5e book lengths. Granted, the Waterdeep guide in
Dragon Heist was only 25 pages long, but that did seem to be pretty light coverage for the city.
That would mean the general geography would be in the player's book (which does make sense; players will want some idea of where their characters might be from, where they're adventuring, and so on). I'd also assume that we'll get some information on the deities of the setting, which wasn't mentioned as being covered, but would have to go somewhere. All of what was mentioned in the article, plus the geography and deities, was covered in the first 281 pages of the 3e FRCS. With some judicious trimming (for instance, we don't need 24 pages on the Dalelands when they will be covered in the other book - maybe just 4 - 5 for an overview instead), that could probably be walked back to 256 pages as well.
Granted, that might just be wishful thinking on my part for 512 pages of FR goodness, but I'm hoping that they won't go below say 196 each.