head to head
Left 4 Dead 2 asks more than Helldivers, and the gap is reads.

Optimization · Practiced Mastery
Twin-stick co-op shooter precursor.

Execution · Practiced Mastery
Co-op zombie shooter with adaptive AI Director.
Helldivers leans Optimization (Practiced Mastery); Left 4 Dead 2 leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Helldivers
Twin-stick aim and stratagem inputs
Left 4 Dead 2
Headshots under horde pressure need aim
Left 4 Dead 2 leans clearly harder on reads.
Helldivers
Scripted enemies, no real reads
Left 4 Dead 2
Versus special-infected ambushes have reads
Helldivers leans a little harder on variance, though the gap is small.
Helldivers
Random spawns and patrol pressure
Left 4 Dead 2
AI Director randomizes spawns and items
Helldivers leans a little harder on optimization, though the gap is small.
Helldivers
Loadout combos and stratagem chains
Left 4 Dead 2
Route knowledge and weapon picks 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.