Without even reading I'm assuming they bought the nearly expired and practically valueless debt from debt management companies for people who they were unlikely to ever be able to collect on for less than pennies on the dollar and then symbolically forgave it on its last days before it was permanently stricken from their credit history and unable to be legally collected on anyway as a PR stunt.

Edit: Read it, they forgave expired debt and got rid of 12+ year old debt of uncertain payment status for the poorest people in their communities.

I just want to highlight Undue Medical Debt[0], the NGO that helped implement this.

They have a fascinating model -- they can buy the right to collect medical debts for pennies on the dollar. That debt would normally be bought by debt-collection companies. But once Undue Medical Debt buys a person's debt, they just... forgive it.

This means they can retire someone's $5,000 debt for a cost in the low hundreds. That might be life-changing for a poor family.

[0] https://unduemedicaldebt.org/