He uses it Perfectly, it's just that He seems to have Abandoned Fighting long ago, hence why He was not just Past Prime but completely not fit for Combat,
Something that many People didn't notice but Moria actually never used a Single Named Attack in entire Thriller Bark Arc,
He only used Utility Techniques rather than actual Offensive Techniques meant to End Someone,
Doppelman = Summoning his Shadow
Shadow Double = Switching Places with his Shadow
Brick Bat = Turning his Shadow into Multiple Bats
Shadow Revolution = Manipulating Shape of Shadows
Shadow Asgard = Absorbing All Shadows
Black Box = Create a Shadow Trap/Prison
None of these Techniques were actual Attacks, so Oda wanted to show us that Moria still have Peak Mastery of his DF but He lost his Fighting-Skills/Style that relies on this DF (Something that was Stated countless times),
I think People are confusing between DF Mastery & DF-Based Fighting Styles,
For example, everyone keep saying that Robin's DF if eaten by a Rokushiki Master would look Insanely Powerful, but it doesn't mean Robin is a Bad User, She completely has Full Mastery of her DF, it's just that She has her own preferred way of Fighting,
Foxy too, He has High Mastery of his DF, or Mr. 5 or Mr. 3 or Wapol or Baby 5 ... etc
It's just that They lack a Fighting-Style (Martial Arts or Sword-Style or whatever) that can combine with that DF for Incredible Results,