Because Slade is a Metahuman.

The experiment the army did on him activated a metagene that granted him a number of abilities.

His Strength, Reflexes and Mental Capacity are increased by a factor 9-10 times in comparison to a ordinary human.

Other Powers include, Superhuman Speed, Superhuman Agility,A regenerative healing factor,deacclerated aging and enhanced senses.

“Confirmed” it how? We need sources or the claim isn’t credible.

I assume you're referring to puffy?

From this wiki

@Jinzo126 I don't think that exists in the DC Universe.

Slade was subject to super soilder experiments that merely activated a metagene already present in his genome. That's why you can't use his blood or other dna to make yourself a super soilder it only worked on him .

@Deathstroke121980 so basically dude ages slower than Superman

Well I wouldn’t go that far^ but yeah like summarised above he is far beyond the levels of most Humans

@BlueDaly I Forgot the Correct Name, but he Got Some Kind of Serum or Experiment that turn him into a Super Soldier.

Drugs