If OP was going to his local record/cd store and not some fringe underground market, he was 100% supporting some record company. Maybe not one of the ones listed, but in the 80s it would have been a company.

By "fringe metal", they're likely talking about mainstream thrash and groove metal. And those bands where on these exact labels;

Reign in blood - Def Jam (1986) Pantera - Atco /WB (1986) Megadeth - Capital (1986) Sepultura - Roadrunner/WB (1989) Anthrax - Island/Universal (1985)

This does read like a slight snark (the step from 'fringe' to 'mainstream' - who wants to be mainstream?). It further emphasises the point that in the 80s and early 90s looking for non-local 'long tail' music as a youth was pretty much impossible. Especially if / when too young for bars and concerts, all you could find had to be signed on mainstream labels. I had the luck in that I could go to local concerts, thus discovering local talent (and buying their cassette tapes). You needed venues and allowing parents for that. Currently listening to several artists with < 500 streamers a month on Spotify. Nothing in the 80s and 90s can compare with that discoverability.

Well, maybe. Around where I'm from, even the chain record stores would stock some unusual music (like metal) if it was locally popular, but the metalheads had their own stores. I think there's still one or two active in Oslo.

A lot of boutique labels are just sub(-sub) labels ran under the big three. Including metal.

Often they’re ran by artists under contract with one of those three big labels and their big sublabels, mostly as a perk/favor to those artists. They have a bit more creative freedom and leeway who they sign, but the money is still kicked upward and the marketing and recording budget still comes from upstairs.