Yes, it's true that the UK economy is very London-centric, but the original poster was talking about the UK as a whole vs the US as a whole. (The flip side of this is that the figures would look better if you compared London to a major US city.)
None of this changes the fact that US software engineering salaries are a poor comparison to use to illustrate wealth disparities between the US and other countries, as they are an outlier.
"another respected data journalist, John Burn-Murdoch, calculated that without London, the UK would be poorer, in terms of GDP per capita, than even the poorest US state, Mississippi."