I don't believe the "corpus" argument that much.

I have been extending the Elm language with Effect semantics (ala ZIO/Rio/Effect-ts) for a new langauge called Eelm (extended-Elm or effectful-elm) and both Haskell (the language that the Elm compiler is written in) and Eelm (the target language, now we some new fancy capabilities) shouldn't have a particularly relevant corpus of code.

Yet, my experiments show that Opus 4.6 is terrific at understanding and authoring both Haskell and Eelm.

Why? I think it stems from the properties of these languages themselves: no mutability makes it reason to think about, fully statically typed, excellent compiler and diagnostics. On top of that the syntax is rather small.