So they may support a single old version because the effort of upstreaming the support would be expensive.
The blog post says that the Librem paid 15 developers full time for 2 years on the project.
That's a huge expense and most of all the customers of the SoC won't be demanding upstream support.
Of course also what happens is that you get into a rabbit hole of patching your fork until it becomes a huge task to reintegrate.