I think the GP has the following causal diagram in mind:
+---------------------+
| Author's competence |
+-------+-----+-------+
| |
| |
+--------------+ | | +---------+
|Implementation| <-+ +-> |Product |
| language | |language |
+--------------+ +----+----+
|
+----v----+
|Language |
|quality |
+---------+
So you are both right; language quality is a function of the language itself, but if we don't yet know much about the language or its author, there is a correlation ("confounding") between language quality and the choice of the implementation language, so if that's the only information we have, might as well take advantage of it.
I am not weighing in here on what specifically the choice of Haskell, OCaml, or Go say about author's competence; I am simply addressing your point that the choice shouldn't matter at all—unless you are convinced that the knowledge of PL theory and design principles is uniformly distributed across all language communities.