gp I'm not aware of the implementations utilized for other Mac password manager apps. I presume most are exclusively utilizing their own extensions, from which there's privacy and convenience arguments to be had.
Additionally, with adoption of AutoFill, I don't believe its use would be limited to password manager apps.
Minimalist, a password manager designed exclusively for Mac and iPhone/iPad, leverages Apple’s AutoFill system. While I don't yet have access to the iOS/iPadOS Orion app (awaiting an invite), Orion on the Mac seemingly doesn't currently leverage AutoFill. This post from Minimalist's help articles speaks towards the benefits regarding security, privacy, and convenience of AutoFill.
