No, like a breakup fee in a commercial contract.

So a subsidy to stop building, instead of a subsidy to start building

No, it's compensation for backing out of a deal. In contract law, once a deal is signed, you're entitled to the "benefit of the bargain" if you perform your end of the deal. A breakup fee allows one side to get out of the deal, while compensating the other side for sunk cost into the project, opportunity costs from not having pursued other deals, etc.

I think you're missing the commenter's point.

...the point being the hypocrisy of exercising that option in the name of removing subsidized power, not the literal financial mechanism.

More like a bribe.