^ Haha, this is definitely the first thing I saw!
The only thing correct were your suspicions.
We are taught to say The only thing correct was your suspicions.
British/Australian English has a very different set of rules from American English regarding plural words. For example, regarding Sweden's soccer team, a singular group made up of several individual players, would be referred to the following ways:
English commentator: "Sweden ARE playing quite well."
American commentator: "Sweden IS playing quite well."
Neither is incorrect, they just look at the structure differently.
As for your OP, that is a bit clunky of a sentence, but it is not incorrect. It is just adding the "extra" part of the sentence that isn't usually needed in casual conversation. Most people would know what you meant if you said, "They can be longer than the average human man." However, we don't usually describe tall humans as "long," so the author felt the need to draw a type of grammatical comparison between an eel's LENGTH and a human's HEIGHT.
