Read the OP_RETURNS. It was the mining pool itself that paid for it by forgoing the fees it could have earned in the mempool. Some 0.018 BTC that was awarded the next block.
Discussion
Plus Mara already did a similar thing using mempool.space's fee visualization before.
Oooh link?
https://protos.com/you-can-now-mine-rather-than-inscribe-a-square-bitcoin-block-image/ But looking at it, mempool might have changed something?
Neat! You have to add audit=false
Indeed they did, but I assumed it was a paid service these days.
So is that a political donation then? Or just art?