Well, there are known stories of "accidental" key leaks: https://www.schneier.com/blog/archives/2016/08/microsoft_acc...

That would still stand out as a sore thumb.

Deliberate introduction of hard to exploit 0days would be precisely how they would do it. All you need is one plant with commit access.

Public key crypto can be broken if the certificate isn't securely pinned on the client. In other words, the adversary could insert their own cert and replicate the exploit.