February, yeah fine. We've seen pretty much all that stuff already, we know it's coming sooner rather than later. The rest? Just a case of throwing darts. We know the factions will get Codexes, we know there aren't that many left to go and we know GW is aiming for 10 inside 6 months. Guessing which order the remaining Codexes will come out in isn't really newsworthy, IMO.
What really makes this looks like a guess, rather than any sort of insider knowledge is the notable lack of actual models mentioned in March and April; we know from 8th edition experience that most existing factions with large plastic ranges are not getting new models alongside their codexes. I'd say Tau, Necrons and Dark Eldar all qualify for that kind of no-model release, and while it's possible that Harlequins could get a Custodes-type expansion, I'd say it's pretty unlikely.
So where are the model sales coming from in March and April? There is no way at all 2 months will pass without significant model releases, and a Blood Bowl team does not qualify as "significant".
Is it only Codex: Knights that's missing from this list? That makes me doubt its veracity somewhat, because Codex: Knights seems like a quick and easy release for GW, so that should definitely be out in the next six months.
Do we even know for sure that there will be a Knights Codex? They are already included in the AdMech book, after all.