Defense in depth means layering security. It's, for example, when you use password hashing but also full disk encryption. That way if someone gets your hard drive, even if they break the disk encryption, they don't get your password in plaintext. Even if they know how to crack the password hash, they first have to get past the disk encryption.
Notarization and signatures aren't two separate measures. They're the same measure implemented two different ways. That's basically useless. If some piece of code is identified as malware then it both gets revoked and added to the malware list, and then they both catch it. If it hasn't been identified then it's neither revoked nor on the malware list.
The things that make it past one also make it past the other. There is no defense in depth because there is no depth. The two measures would have to operate based on a different principle in order to achieve that.
