They don't have the mass necessary for continuous fusion, but they're still massively radiating in the infrared (heat) from slow gravitational collapse and they have plasma surfaces and strong magnetic fields like a star.
Gas giants, in contrast, have rocky cores down deep, massive layers of gaseous atmosphere, and while their magnetic and gravitational fields are strong compared to small stony planets, don't compare to those of Brown Dwarfs.
So are Brown Dwarfs stars? Well, I'm still bummed about Pluto getting demoted, so I don't wanna talk about it.