A bigger team doesn't create buggy software because it contains below average coders.
It creates buggy software because the team members have to coordinate with more people.
It creates buggy software because the team members have to coordinate with more people.
But it must, by the very meaning of "below average coders" - it's implied that we're rating hypothetical coders on software development skills, which include the ability to make correct decisions (both at code and architecture level).
> It creates buggy software because the team members have to coordinate with more people.
Those are not independent variables. The bigger the team, the bigger coordination problems, the more bugs. But also, ceteris paribus, the worse the coder, the more problems there are coordinating with them, thus the more bugs. And also, ceteris paribus, the worse coders are on average in the team relative to global average, the more of them are needed for a project, which makes the team bigger and coordination more difficult. Etc.