It should be based on the number of successes relative to the amount raised or ROI and that picture is quite different. In that sense London is way behind SV.
VC in London is harsh, both for the start-ups and for the VCs. What does happen is that a company manages to stay alive long enough to raise a secondary round in the USA, but then you can't really make the original claim in the TFA.
We are using different meanings for the same phrase. SV is the best place to raise, IF YOU CAN AND ARE WILLING TO RAISE THERE. But not everyone can, nor does everyone want to. And of the locations that are not in the US, London dominates.
And hell, Americans raise in Boston, Seattle, NYC, and so on. Not even all Americans move to SV, let alone people who may not even get a visa to enter the US.