Although it’s more likely they’re “independent” but working for some company using “independent contractors” as a way to avoid offering benefits and following labor laws.

That’s called piece work, those are proles. Unusual in the trades afaik. In thoroughly unionized work like plumbing they’d be scabs. They should join their UA local 1 and get union jobs instead of scabbing.

A (truly) independent plumber is a worker that owns their own means of production. Literally the ideal.

They are an artisan, with different material interests than proles.