Think of it as a one-time license fee to white label the Medium tech. Pretty cheap, imo. Surprised they didn't make it a recurring fee, tbh.
Out of curiosity, what benefit do you see getting from having the content be on your own domain vs. Medium itself?
It's probably just me being quirky, but when I'm reading a Medium article on someone's custom domain, I get slightly annoyed that it's a little harder to get back to Medium itself to get to the rest of my reading list.
If you decide against paying the fee, I doubt I'm alone in saying that I don't judge the content itself on whether it's on medium.com vs. randomtechguymusings.*.
Publish good, thoughtful, valuable content and your work will speak for itself regardless of domain name.