It's not surprising if you think the non-mainstream intellectual community is large enough to contain excellent writers that were currently under-utilized and under-financed.
Excellent writers could have been ignored by the mainstream market previously if they were outside of the norm and sometimes found in baskets with bad apples (e.g. "alt-right"). The non-mainstream is actually a collection of smaller minority groups -- it tends to be the mainstream that is more homogenous. It doesn't surprise me that Substack has found success by not treating the non-mainstream as equally worthy of scorn.
It also won't surprise me when they start to attract more writers from the mainstream. The mainstream can sometimes appear to be totally homogenous in their hatred of liberal content policies but this is mostly due to the efforts of a small minority. If the offering is good, others will slowly come aboard.
As another commentator noted (https://news.ycombinator.com/item?id=31084791), Substack's policy is politically more in line with neoliberal complaints of censorship than the complaints from those in the alt-right. (Their comment is better than mine since it is a more direct response to the core of your comment, which could be read as suggesting that use of Substack points towards sympathy for the "alt-right".)