From what I understand (also FTA: “The Unity IAP team found themselves having to harden their code against random data”) the fix wasn’t to read the documentation and do the right thing, but to go deeper in on “if we step carefully through this minefield, we probably will survive”.
And Unity’s customer is happy with that!
I was expecting something much more interesting from the title, or mere posting of the story. It seems it was picked because of the cost/impact of the bug rather than the technical surprise/complexity.