No. Users can still disable Gatekeeper for complete control, just as they can disable Secure Boot on PCs, and just as they can disable Google Play Protect and enable "unknown sources" on Android.
Nitpick: Secure boot is about the firmware protecting the machine at boot time against unauthorized OSes or OS-modifications, establishing trust in the boot-media.
App-signing is about protecting the user within an already booted OS, trusted or not.