Honestly, I didn't consider this. I just looked up the KDP Select exclusivity agreement:

1 Exclusivity. When you include a Digital Book in KDP Select, you give us the exclusive right to sell and distribute your Digital Book in digital format while your book is in KDP Select. During this period of exclusivity, you cannot sell or distribute, or give anyone else the right to sell or distribute, your Digital Book (or a book that is substantially similar), in digital format in any territory where you have rights.

I suppose it is a gray area because the Medium serial is not a digital "book", but a series of posts.

To be safe it is probably better to remove the Medium serial before enrolling in Kindle Unlimited. Or not enroll in Kindle Unlimted.

More experienced self-publishers can probably give you a better answer.