The monsters don’t understand game mechanics.
“Disadvantage on attacks” isn’t something that anyone in the actual fiction of the game comprehends. Of course they understand “if I close my eyes, it’s harder to hit stuff”, but they don’t have the concept of “disadvantage on attacks”.
There is no reason for the creature to know how it works.
That said, why would the monster know the armor is going to pulse when it strikes someone else? The feature says it emits the distracting pulse when it attacks, and so unless that creature happens to know from previous experience fighting the artificer, it probably should be a surprise the first time the artificer’s armor emits a distracting pulse. There’s just no reason for any target of the feature to just know ahead of time what’s going to happen.
And since the creature doesn’t know how it works, it shouldn’t be a part of their decision making when attacking. I’m not saying who they have to attack, but you should just make that decision as though you had no idea how the armor works, at least until they figure it out. So your player is basically on the right track. There is no reason for the creature to know what’s going on until the armor distracts them. Once the armor distracts them once, then the creature can think “hey that was distracting, let’s kill him”.
As for the range of the feature, the maximum range is something like “as far away as you can get before your next turn”. No range is stated, so technically there is no range. Though in most circumstances you won’t be able to get that far before your next turn.
This is quite similar to this Q&A: Do enemies know that a character is using the Sentinel feat? In my answer, I write:
Characters in-universe have no concept called the Sentinel Feat.
Rather, they experience its application in the fiction. Most of the rules and features in the game do not correlate to tangible things in the fiction of the universe. Things like spells do, somewhat, but the Sentinel Feat translates to "this person is really good at pinning you down in a combat" - and this is something that is learned through being pinned down in melee combat.
Now, if a character with the Sentinel Feat was widely known for being good at pinning people down in melee combat, enemies who had heard of this character's skills might be able to strategize against this tactic.
In response to this question: Is there a way to ask in game (i.e. in a non-meta way) what a character's class is?
, T.J.L. nicely lays out the distinction between game mechanics and the fiction of the world:
class is a metagame construct: from the characters' perspective, it doesn't exist. There is no good way to determine "class" as a hard fact for the character, because a particular set of abilities does not cleanly map to the character's identity and societal position in-world.
To put it a different way (using D&D 5E terminology)...
You have a two characters who wear heavy armor without discomfort, swing a greatsword with skill, and call on the powers of a deity to enhance their abilities and destroy their enemies. Are you dealing with a War Cleric or a Paladin?
You have two characters who wear medium armor, wield a longsword, and cast arcane spells. Are you dealing with an Eldritch Knight Fighter, or a multi-classed Fighter/Wizard?
To emphasize the difference even more... all four of these characters, if asked in game (without metagaming), may call themselves a knight.