This fish is not "fully warm-blooded". As described in the article, it does not target a specific body temperature like mammals or birds. It just uses its muscles to warm itself, like some species of snake and insect do.
Does targeting a specific temperature determine if you're classed as warm blooded?
Relevant article snippet:
"Some other types of fish, such as tuna, have similarly designed blood vessels in certain parts of their bodies, allowing for “regional endothermy” — warm-bloodedness that’s limited to certain organs or muscles, such as the eyes, liver or swimming muscles.
But the opah is the only fish scientists know of that has this design in its gills, where most fish lose the majority of their body heat to the surrounding cold water. By warming up the blood in the gills before it goes anywhere else, the opah achieves not just regional endothermy, but whole-body endothermy, according to the paper’s authors.
Testing showed that the opah is able to maintain a core body temperature about 5 degrees Celsius warmer than the surrounding water."
