Walking to the wingtip without making move the plane (Mustang) is barely an indicator of the wing strength. I'd say it didn't move because the landing gear is set so much more apart from each other in a P51 than in a Spit that made the difference here.

One thing is fact which in turn is exploited today on purpose for modern fighter design but which also extends to other domains:

The less stable a device is the more prone is it to change its state. This principle can be exploited in a beneficial way. If you make something instable it is more easier to move around. For instance designs like the Eurofighter is instable and only kept on course because of computer software. This inherent instability allows to be more manoeuverable than a stable plane because anythings stable will tend to maintain its current status and is highly unwilling to assume another state (that is another attitude or flight direction).

So if the Spit is as manoeuverable it is likely on the edge of stability and thus somewhat nerveous.