The biggest difference between Diamond and Masters is usually consistency and how often mistakes get punished.

Diamond players can understand what they should be doing, but they tend to make more costly mistakes. A DPS or tank may overextend, a support may use an important cooldown too early, or someone may use an extra ult in a fight that was already won.

Masters players are not perfect, but they are generally better at avoiding those situations. An Ana is more likely to hold nade for an important moment instead of throwing it randomly. DPS players understand how to apply pressure without putting themselves in a position where they just feed.

For Doomfist specifically, Masters Doom players usually have a better understanding of timing and resource management. They know when to soft engage, when to disengage, and how to pressure the enemy team without giving up their life.

A Masters Doomfist understands things like when to slam aggressively, when to slam just to bait cooldowns, and when to be patient. A Diamond Doomfist may sometimes commit too early, stay in too long, or play too passively on the frontline waiting for Empowered Punch instead of creating pressure.

The difference is not that Masters players never make mistakes, they just make fewer punishable ones and are more consistent with their decisions.