In reality, there's no blame to assign when tackling tech debt. Real projects are messy. Folk defer work for short term gains at the cost to their future self. Growing tech debt is just an eventual engineering problem to tackle like anything else. Best practices are distilled wisdom intended to keep those sorts of problems from growing out of control.
Re: the competency of the SO core devs, they very well could and probably are solid within their domain. Not knowing anything about them though, I think it's possible that they're a bit siloed off in their knowledge. If a lot of their core team were there from the beginning of the project and were relatively inexperienced at the time, they may not have ever invested their time into developing test tooling. In some ways it's very impressive that they're able to keep that large of a project going with so little testing infrastructure.
Edit: added missing “is”