I stand corrected. For some reason, I was thinking they used the actual Windows Defender API, which can be called programmatically from third-party applications, but you’re correct, it was a driver loaded at boot. Microsoft isn’t at all at fault, here.
Nope. It’s a lower level kernel API that has to be accessed at boot via a driver. The API I was thinking of - and I use the term “thinking” loosely, here - is an API that userspace applications can take advantage of to scan files after boot is already complete.