"... how do they know that these various nodes are correlated with one another ..."
The OP alludes to this:
"... and the fact that someone runs such a large network fraction of relays “doing things” that ordinary relays can not do (intentionally vague), is enough to ring all kinds of alarm bells."
... and the OP is "intentionally vague".
I, also, am very interested to know how they correlated them and what the interesting behavior was that they exhibited ...
The OP alludes to this:
"... and the fact that someone runs such a large network fraction of relays “doing things” that ordinary relays can not do (intentionally vague), is enough to ring all kinds of alarm bells."
... and the OP is "intentionally vague".
I, also, am very interested to know how they correlated them and what the interesting behavior was that they exhibited ...