I know that your abl/bootloader/fastboot doesn't have any flashing commands, but I didn't check if it had a boot command. It's always better to check a new image by booting before flashing.
Code:
fastboot boot Magisk123456.img
If it works or if it doesn't then you can proceed to flashing.
Code:
edl /lpatched.bin
edl /u /w /pboot_b Magisk123456.img
edl /z
Note: It could be boot_a or boot_b, The last I saw your system it was on b active.
You can do a "fastboot getvar all" or "edl /u /g" to see which slot is active.