he was wrong
The fact that one is a ranged attack with a 10' range, and the other is a melee weapon, seems inconsequential to me. Both are non-magical forms of attack. The argument was "it's magic" in response to things like magic missile and fireball. Not "it's ranged". Or would you be fine if the fighter threw a dagger and did auto-damage on a miss with a thrown dagger? I doubt it.
The fighter ability is being described like splash damage. But, everyone's upset about describing it that way. For some reason they are fine with a small vial of liquid hitting every single spot across a 15' area exactly evenly without any ability to dodge it or have armor prevent damage from it, but they're not OK with a fighter hitting everyone in a single 5' square with their big weapon, with at least a glancing blow.
And I don't get that. Throw a bottle of something and watch it break - the splash almost always goes in the direction of the momentum of the bottle, and never evenly disperses across a 15' area.
I'd say hitting everyone in a five foot area with a sword, with at least a glancing blow, is more realistic than alchemist fire hitting everyone in a 15' square exactly evenly with no ability to dodge it or have plate armor prevent damage from it.