That is all true. At the same time here the groups were allowed to use the language of their choice. Presumably they chose languages they felt they were competent in.
Of course an expert of a given programming language can write much better in it than a novice. But a comparison like this is not necessarily about comparing top-programmers in every language, but average programmers, because we want to know results that are true "on average" .
The author does note he "knew (they) were highly competent". So they were not exactly novices in their language of choice. Writing a compiler is not a task for novices in general.
Of course an expert of a given programming language can write much better in it than a novice. But a comparison like this is not necessarily about comparing top-programmers in every language, but average programmers, because we want to know results that are true "on average" .
The author does note he "knew (they) were highly competent". So they were not exactly novices in their language of choice. Writing a compiler is not a task for novices in general.