Yes, Substack is blogging, and it's totally valid for a blogging system to be better at publishing one kind of writing.
What's wrong with Substack, and why it will ultimately need to change or be replaced, is that they require writers to use their editor.
That's lock-in. Not interop.
And interop is part of the philosophy of blogging imho.
As a writer, I use the writing tools best suited for me.