The warden doesn't care about a mob attacking it if it focused on another mob, even if it is out of reach. This could be cheese of the warden as it could aggro on a zombie or any mob out of reach and a simple iron golem will kill it without fighting back. I had tested this on the player and it seems like it will only switch to the player if the player hits it while attacking another mob, otherwise any mob like an iron golem or zoglin could kill it while it is focused on that out of reach zombie.