[JNRowe]

Guessing…

Being a YC company¹, presumably the overlap with people who are aware of both is quite high. They also pay some "big names" an advance to write², and again the overlap of chosen people/interests seems likely to align to some extent.

FWIW, there was a time when the front page basically felt like a Medium indexer, and I imagine this cycle will eventually be the same as the platform changes/dilutes/grows(delete as appropriate).

¹ https://www.ycombinator.com/companies/substack

² https://blog.substack.com/p/why-we-pay-writers