I don't buy it. Or rather, I am willing to believe that some team at Apple has convinced itself that this makes sense, but they're wrong.
In particular, the security boundaries are nonsensical. The whole model of "notarization" is that the developer of some software has convinced Apple that the software as a whole (not a specific running instance) is worthy of doing a specific thing to the system as a whole.
But this is almost useless. Should Facebook be allowed to do various things that can violate privacy and steal data? What if the app has a valid reason to sometimes do those things?
Or, more egregiously, consider something like VSCode. I run it, and the fancy Apple sandbox helpfully asks me if I want to grant access to "Documents." The answer is really "no! -- I want to grant access to the specific folders that I want this workspace to access", but MacOS isn't even close to being able to understand that. So instead, one needs to grant permission, at which point, the user is completely pwned, as VSCode is wildly insecure.
So no, I really don't believe that MacOS's security model makes its users meaningfully more secure. At best, the code signing scheme has some value for attribution after an attack occurs, but most attacks seem to involve stolen credentials, and I bet a bunch just hijack validly-notarized-but-insecure software a la the VSCode example.