There's literally a name for the GI illness escolar causes: keriorrhea. It's caused by the indigestibility of wax ester-containing oils in the muscle tissue of the fish; it's a phenomenon similar to those "fat-free-fat" potato chips they used to sell. The "histamine" thing is a reference (the author doesn't know it) to a problem (scombroid poisoning) caused by improper storage of many different kinds of fish, not just escolar --- tuna and amberjack cause it too. Scombroid poisoning happens because, above a certain temperature, histadine in the fish flesh is enzymatically converted to histamine.

Happy to help! Would have been happy to help the author of this bad article, too. Oh, well.