Ian Petrow defines the Subconvexity Problem as "Prove non-trivial upper bounds for L-functions on the critical line" [1]
Given that, it seems fair-ish to say that Nelson solved the Subconvexity Problem. You just have to understand that the problem is really a family of problems of increasing hardness (e.g. prove tighter and tighter bounds), and solutions more powerful than Nelson's may come later.
By that definition, I agree, if you add the qualifier "Nelson solved the subconvexity problem for for automorphic L-functions."
I still think it's vague to talk about "the" subconvexity problem without specifying what variable you want the bound to be subconvex in, but, really, who am I to argue with Ian Petrow..!
Given that, it seems fair-ish to say that Nelson solved the Subconvexity Problem. You just have to understand that the problem is really a family of problems of increasing hardness (e.g. prove tighter and tighter bounds), and solutions more powerful than Nelson's may come later.
[1] https://www.ucl.ac.uk/~ucahpet/SubconvexityIntro.pdf