It could come from inactive subscribers — there are a lot — and from the portion of the Medium fee that goes to Medium. That’s the pot of money that pays for their other costs: staff, admin, development, competition prizes, bonuses, etc.
They’ve always been able to find money when they want to for such things. It’s possible that a further cut to writers’ fees could happen, but there’s been nothing spent on competitions or bonuses for a while, so there might be a pot for this. I hope there’s no further erosion of writers’ fees.