Possibly, I don't remember Hyde Park Corner being silly busy, but it probably was more than normal Sunday busy. The riders finish the 100 and are then guided in to Constitution Hill. Green Park is full of festival. Once you're ready to go home, it's Constitution Hill on to HPC as the leave route.
In my weave around for the riverboat pier we were directed south of the river between Westminster Bridge and Waterloo Bridge (well, the boat signs disappeared as soon as we were south of the river, so I got us north again at the first opportunity) and traffic was gridlocked and grumpy along York Rd south of the river, so I wouldn't be surprised if there was similar on the main arteries on the north side. It really wasn't Sunday level traffic.
As well as the cycle route, Embankment was closed for people to ride back to Docklands, but I don't know how long for. It can't have helped disruption though.