Given that Magic Missile functions like a special AoE spell (one damage roll for all missiles), it's non-interaction with Mirror Image falls in line with other AoE damage.
I call shenanigans on this as well. I have never heard such a ridiculous ruling from a "Lead Rules Designer" in my entire life. And I mean that literally and I am not exaggerating. Here is the text of Magic Missile:
Magic Missile
1st-level evocation
Casting Time: 1 action
Range: 120 feet
Components: V, S
Duration: Instantaneous
You create three glowing darts of magical force.
Each dart hits a creature of your choice that you can see within range. A dart deals 1d4 + 1 force damage to its target. The darts all strike simultaneously, and you can direct them to hit one creature or several.
At Higher Levels: When you cast this spell using a spell slot of 2nd level or higher, the spell creates one more dart for each slot level above 1st.
Here is Fireball:
Fireball
3rd-level evocation
Casting Time: 1 action
Range: 150 feet
Components: V, S, M (a tiny ball of bat guano and sulfur)
Duration: Instantaneous
A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame.
Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.
The fire spreads around corners. It ignites flammable objects in the area that aren’t being worn or carried.
At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.
There is a subtle difference that may have escaped the esteemed Mr. Crawford's notice. The difference where one is an area of effect attack and the other is not. The difference where one does 1d4+1 damage per dart and the other does 8d6 damage to everything within its radius.
Here is another good spell, Hold Person:
Hold Person
2nd-level enchantment
Casting Time: 1 action
Range: 60 feet
Components: V, S, M (a small, straight piece of iron)
Duration: Concentration, up to 1 minute
Choose a humanoid that you can see within range. The target must succeed on a Wisdom saving throw or be paralyzed for the duration. At the end of each of its turns, the target can make another Wisdom saving throw. On a success, the spell ends on the target.
At Higher Levels: When you cast this spell using a spell slot of 3rd level or higher, you can target one additional humanoid for each slot level above 2nd. The humanoids must be within 30 feet of each other when you target them.
Do you see how Hold Person is also an area of effect spell that effects everything within its radius? No? That's because it isn't an area of effect spell. It talks about targets just like Magic Missile does. Because Magic Missile isn't an area of effect spell either.
Sorry, but even "Lead Rules Designers" can make mistakes. The Magic Missile ruling is one of his biggest.