In this specific case, the NFT owners had, at some point in time, listed their assets for sale on OpenSea. The issue was that you could re-list (e.g., at a higher price) and the transition would be stored in OpenSea's off-chain db, but not complete the on-chain cancellation (supposedly because the user didn't want to pay gas fees, and/or the UI was confusing).
OpenSea would show the transaction cancelled/relisted in their UI, but the on-chain contract still had the item for sale using the original price. If you know where to look, you could skip OpenSea's UI and interact with the contract directly, thus yielding a handful of Bored Apes for a fraction of their current price.
Clearly folks exploited a loophole, but at the same time "code is law" and all of that, and clearly OpenSea's fault for not preventing this from happening in the first place, so it doesn't surprise me they are stepping in and covering the costs.
This thread has the full details: https://twitter.com/yakirrotem/status/1485559864948629512