I've got an unusual sale on Alamy. Under the use details, it tells me "Editorial website and app multiple use, in perpetuity; Infringement."

I assume this means a buyer was wracked with guilt over stealing an image and decided to come clean and buy it for real?

More likely it was brought to the attention of Alamy in some fashion and then they had to pay for their previous illegal use.

For example someone was selling calendars and another user noticed their image in use and told Alamy, and then Alamy saw all 12 months were from Alamy, including one of your images and forced them to pay.