What HN-like forum software is substack using?

I was recently introduced to substack[1] and was immediately impressed by the threaded discussion software that they are using.

Here is an example:

https://frenchpress.thedispatch.com/p/politics-is-a-jealous-god/comments

What forum software are they using ? Is it homegrown/in-house ? Other than lobste.rs, it is the nicest clone of HN I have ever seen ...

[1] https://substack.com/