They are also independent of corporate conglomerates, unlike DC and Marvel.
I might point out, that much like Marvel and DC, prior to their acquisition by larger companies, that the company itself is the corporate conglomerate. I think one of the best arguements for they’ve maintained an independent status is how they structured image, such as how McFarlane is a separate entity which is published by Image, but their makeup is just as convoluted as any other big company.
Outside of that, they publish huge and well known properties. Spawn is a huge mainstream IP, then even just tossing Walking Dead into the mix, I can’t even imagine considering it an indie publisher any longer.
It’s like a band. They may be amazing still, but they lose their street cred and start being “sell outs” once they’re successful.