(posterity) Sure the primary OS/Bootloader key probably falls under vendors, but wasn't much of the core systems (phone, play, browser,etc) moved to be under Googles control due to the vendors being sloppy with providing updates for devices in the olden days? (And as such this key would be the thing that the os/loader grants privilegies to).
Edit: Noticed the comment about them being lowend(mediatek?) certificates.
My understanding is that with v3 google started requiring that app developers send their private keys to google, as a requirement for inclusion in the play store (using the 'Play Encrypt Private Key (PEPK) tool').
In light of that I guess it wouldn't be shocking if there were similar requirements for vendor's platform keys.
v1/v2/v3 APK signing has nothing to do with the Play Store requiring Play Signing for newly published apps. App bundles / split APKs also don't inherently have to be used the way they're using them. Entirely possible to use them outside the Play Store with your own signing keys. v3 signing is just v2 with key rotation support. v2 is proper whole file signing instead of v1 which was just the flawed JAR signing system.
Edit: Noticed the comment about them being lowend(mediatek?) certificates.