What reasoning do you have to say that open sourcing it would amplify problems because it allows it to be fixed by people without deep knowledge of the codebase

What reasoning do you have to say that open sourcing it would amplify problems because it allows it to be fixed by people without deep knowledge of the codebase? So confused by how enabling someone to see the source code will somehow hurt the code that would still be maintained by apple. It would still be up to them on whether or not to include a fix that was proposed by someone that viewed their code and tried to fix it. Also, I think that fresh eyes on a problem can be quite beneficial. I believe a tunnel vision can occur when one group of individuals are trying to fix the same problems for years.