There is certainly not enough information in that quote to say one way or another what the bug was. I've definitely seen concurrency bugs under load because data that wasn't supposed to be shared actually was, e.g. I posted this serious GitHub bug in a comment above, https://github.blog/2021-03-18-how-we-found-and-fixed-a-rare....

Obviously sessions should be independent and not sharing data, but that's why it was a bug.