Medium's a great place to write, but it's not a great place to collect emails — their subscription system is fairly low-functionality, and is missing a lot of the features that writers need to build a sustainable business around their writing and subscription base. But that doesn't mean they shouldn't publish on Medium; they just should make sure that they're collecting emails in a place that's more suited to the task.
Thankfully, Buttondown makes this easy. You can embed a Buttondown subscribe form in your Medium posts simply by dropping your newsletter's URL into the Medium editor.
I think a working demo is worth a thousand words, so here's a link showing how Buttondown's subscribe form looks in Medium!