Sublicensing means giving a copy of the license to third parties. (both for free and for money, depends on how you want to do it).
For example, a university can legally buy the license to use software, then “sublicense” this software to their students (either for a fee or for free), meaning that the students then receive the software.
Of course, in most cases, a sublicense is part of a regular contract (for example, schools and universities often purchase a specific “educational license” for software like Photoshop.
But sublicensing something means “re-selling” the license to a third party.
If you sell me the right to your story for a book and include the right to sublicense it as I see fit for derivative works, this would mean I would be allowed to sell the book and “sublicense” it to a translator who writes a translation for me, which I then also throw out into the market.
And again, I’m not accusing Medium of doing anything of the likes right now. And they did give us the promise to protect our rights. But once you sign a contract that says “they can if they want”, then there’s no legal obstacle in their way once they decide to forfeit their prior promises.
If they have you sign a contract that says “you give us the right to take your firstborn child” then it is unimportant if they anywhere write “We’d never ask you to give us your firstborn. Pinky swear.”
They still can ask you a year later out of the blue, and if they do, you have to obey. That’s how the law works.
A sentence like “you are liable in case X…” is legally binding. A sentence like “We’d never do that. We’re honest people.” is not.