This checks out logical speaking.

The FANG code basis are very large and date back years might not necessarily be using open source frameworks rather in house libraries and frameworks none of which are certainly available to Anthropic or OpenAI hence these models have zero visibility into them.

Therefore combined with the fact that these are not reasoning or thinking machines rather probabilistic (image/text) generators, they can't generate what they haven't seen.

That's why coding agents usually scans various files to figure out how to work in a particular codebase. I work with very large and old project, and Codex most of time manages to work with our frameworks.