That allows them to avoid the problem, but why introduce the problem in the first place? Why not use something that is intended to be used as a crypto library?
They had an existing API to comply with[1]. NaCl's API doesn't cover everything they need. Their choices were down to 1. Use OpenSSL (or a fork of it, which BoringSSL is) 2. Roll their own crypto. They chose #1 on all other platforms and stayed with #2 on the platforms they control (I might be mistaken here, perhaps they wrap some OpenSSL derivative on their own platforms too). And that in my opinion is a reasonable choice, given the constraints.
[1]: https://developer.apple.com/documentation/cryptokit