Hey, so when I asked this question in the Discord a while ago, I got an interesting answer.
Manifest Armor inherits some stats, but not all of them - mainly it inherits the non-special ones, and it applies as if the Manifest Armor is a player. In your case, you, yourself are wearing +level of Manifest Amor and +% inc minion damage. These things effect your Manifest Armor through you. However, on the manifest armor’s version of what you are wearing, they effect the Manifest Armor’s minions (which is probably won’t have). So it doesn’t get double counted.
It is still worth having the stat though as %increased minion damage is multiplicative with everything that is on the armor (so that means if you have 1000% increased minion damage and 1000% increased damage, they get multiplied by each other). That’s how Manifest Armor is probably one of the highest DPS abilities in the game at over 100 million dps if correctly built.