Hera closing with Herakles, Jess (Collins), 1960

The passage is a bit like the US Constitution. By that I mean that it may be interpreted to suit the purposes of a great many people who are at odds with one another. For instance, nothing prevents someone from saying that the essential definition means: once depressed, stay depressed. Of the passage relating to Coleridge there is no doubt: all you have to know is that Coleridge was the great intellectual among the Romantics, the great thinker. But an interesting and further complicating key is provided by the phrase “isolated verisimilitude.” Verisimilitude means “having the appearance of a truth; probable,” so that Keats is saying something like this: “Coleridge would pass over a probability that someone else would accept as the truth because Coleridge is not content with appearance or probability.” If we add to this the idea of isolating, which implies distinction or differentiation, we can’t help but think that Keats has searched the penetralium of mystery at least long enough to isolate a probable truth that is, unto him, sufficient. And this is a far cry from the non-isolating attitude that most of us associate with negative capability.

I do not associate a non-isolating attitude with negative capability, so I may fundamentally misunderstand Ruefle's concern.

Distinction and differentiation can be helpful, especially when accurate. To isolate one thought from another thought, to separate assumption from conclusion, to perceive pieces of the whole can enhance our understanding of the whole

Aristotle argued that what separates a journeyman from an artist is a keen awareness of each element of the poetic process and an ability to explain - and therefore influence - how each element contributes to the final result.

Most of human progress is a struggle against the current day's orthodoxy.  For the last half-century high culture has worshiped at the altar of relativism, uncertainty, and doubt.  This orthodoxy condemns fine distinctions as innately hypocritical or self-interested or stupid.

For any thesis there are many antitheses.