I see mostly the same thing. Frontier have said that all versions of Elite will be able to view the conclusion to the Azimuth Saga, though 'view' is an interesting choice of words but that's not atypical for Frontier. But even so, Horizons players can't view onfoot stuff so the way it reads to me, with the code merge happening after U13, onfoot Thargoids seem to be ruled out, given what Frontier have said, at least until after the code merge, which seems to indicate that Horizons players will see the onfoot stuff, but the code merge seems unlikely to be the trigger for it, as it seems as you say, that would be left to U14 to introduce onfoot Thargoids.
But the way Frontier are building up the conclusion with statements like 'the galaxy will never be the same again' leads me to believe that if it relates to Thargoids, it's going to be Capital/Mother Ships, though that suggests more of a continuation of the Azimuth Saga, so maybe they will be gone for a bit? Which could lead to the introduction of the Guardians, but again, that might be left for U14 to begin the new narrative..
TL
R - Onfoot Thargoids seem ruled out by Frontier allowing all releases of Elite to view the conclusion of the Azimuth Saga - unless the code merge happens with U13, which Frontier have said it isn't. What will happen instead of onfoot Thargoids is much harder to determine.