[kccqzy]

Encapsulation.

This is our code. That is their code. Depend only on the interface of their code and not the implementation. You can look at their code for curiosity but don't depend on the implementation of their code in our code.

Then you don't care what subset of the language their code is written in.

[CodeMage]

This fails to address the question you replied to. When "our code" is memory-safe and "their code" isn't, it's still "Our Product™" that ends up with a CVE that costs us money and reputation, because users don't care about whose code it was.