Different mileage.
Or I'm not clear enough.
That quote is a necessary but not sufficient condition; otherwise most sword and scorcery novels would be space opera. (Conan is definitely a larger than life Barbarian doing larger than life things. Ditto Fafhrd and the Gray mouser.) Even within SF, most action heroes are at least somewhat larger than life.
Let me try again:
SF is the genre. Literature of ideas. (No problem there, right?)
Adventure SF is a type of SF where the narrative focuses on action adventure ("somebody having a bad day").
Space Opera is a sub-genre of SF where everything is amped to eleven but it stays coherent.
Differences?
A good adventure SF novel can have the hero making his way across an alien planet, fighting off monstruous creatures, slavers, pirates, and religious fanatics. (Farmer's GREEN ODYSSEY, Harrison's Deathworld 2.)
A space opera novel will have the hero systematically making his way through an entire galaxy of slavers, drug dealers, and pirate fleets. Each challenge will be overcome in a different fashion, each challenge will be an order of magnitude harder. What the hero does, few can even aspire to do. And he'll do it in style.
(LENSMEN)
Scale makes a difference. (STAR TREK doesn't usually go big. SFX get too expensive.
)
Intent makes a difference; Space Opera is for fun, but it still follows the rules of SF. (Which Star Wars doesn't.)
To my mind, a lot of what people think of as Space Opera (including Wilson Tucker back in 1941 when he coined the term) is actually just bad adventure SF rather than the real thing, which requires *discipline* and consistency.
But as you suggested, real Space Opera is a hard fit in this topic since pessimistic space opera is practically an oxymoron. On the other hand, pessimistic (even nihilistic) adventure SF is common.
So, to get back on track, I'll offer up this:
The ultimate pessimist believes each problem solution only spawns bigger problems and at the end, the sun goes nova.
The ultimate optimist believes all problems have a satisfactory, happy solution and, if you wait long enough, somebody else will solve it for you.
Everybody else fits somewhere between those two extremes.