Pixel 4a : adb detects device but fastboot command doesn't work
I have now idea why this worked. Before running fastboot commands rebooted device in download mode:

adb reboot bootloader to get into fastboot
fastboot devices shows device serial
fastboot flash boot patched.img works :)
fastboot reboot to get back to normal device state
Device is rooted and I see su permission requests

Note: Some devices of LG and Samsung lack fastboot functionality.