head to head
Neither is simply harder - they are hard in different places. Call of Duty: Modern Warfare 2 demands more execution; Sea of Thieves demands more variance.

Execution · Competitive PvP
Blockbuster 2009 sequel famous for killstreak chains and frantic multiplayer.

Reads · Competitive PvP
Open-world pirate game with PvPvE ship combat.
Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP); Sea of Thieves leans Reads (Competitive PvP).
Call of Duty: Modern Warfare 2 leans clearly harder on execution.
Call of Duty: Modern Warfare 2
Fast time-to-kill rewards raw aim and reflex
Sea of Thieves
Ship sailing and naval gunnery require hands
Sea of Thieves leans a little harder on reads, though the gap is small.
Call of Duty: Modern Warfare 2
Outguessing human rotations and flanks
Sea of Thieves
PvP ship fights are pure mind games
Sea of Thieves leans clearly harder on variance.
Call of Duty: Modern Warfare 2
Spawn flips and killstreak randomness
Sea of Thieves
Server encounters, weather, loot vary
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 2
Killstreak economy, loadout and map knowledge
Sea of Thieves
Routes, loadouts, ship setups solvable
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.