Why would these actors want to publish on Substack in the first place? They have their own platforms already. Alt-right content is highly "meme" based (e.g. the whole thing with frogs and 'Kekistan', or the Qanon LARPing), it doesn't do well on a platform focused on long-form texts with serious intellectual interest.
I'm not trying to promote their blog and am not a fan of it and won't link it, but I know of at least one Substack blog by one such actor who indeed makes their blog highly meme-based. The fact that you can insert arbitrary inline images in blog posts and write whatever text you want near them is pretty much all you need.
One obvious answer is revenue. Getting money in is a real struggle for extremists, who tend to get banned from traditional platforms. Think of it as like paying membership dues.
