That's not a current fastboot because it doesn't know how to turn 'boot' into 'boot_a' or 'boot_b' based on the current slot.
If you can't upgrade to the current fastboot, you can try one of these options.
1. Find your current slot and boot that:
fastboot getvar current-slot # will be 'a' or 'b'
fastboot flash boot_a magisk_patched.img # or boot_b
2. Boot the patched image and get Magisk Manager to do the permanent installation.
fastboot boot magisk_patched.img
# use Magisk Manager to install (direct)