Why do you believe that?
Sure, cosmetically, there is still quite a bit to do. But functionally, the stadium would have been usable by mid September but for the issue with critical safety systems identified by the first test event. The interior bowl is nigh on complete. Likewise the roof. I've seen comments to the effect that the seats aren't all in and the pitch hasn't even been laid yet. Well, both those jobs can be done in a matter of days. And there are still 4.5 weeks until the Liverpool game. If you've ever followed a construction project closely, you'll know that, more often than not, the building looks far from finished until the final few days. Check out pictures of Atletico's Wanda Metropolitano stadium at the same stage of construction last year. You'd have said exactly the same about that.....that there's no way it would be ready for a mid September opening. But it was. So would new WHL have been but for the aforementioned systems failure.
A huge, complex construction project on a tight schedule overrunning by a couple of months is most definitely not a fuck up of major proportions.