Woa is this a typo? I know little about England, and I hadn't realized that "keeping up with East German cities" is considered a success. I mean isn't East Germany rather well known for not keeping up all that well with the rest of Germany either?
All the data is from the OECD using comparable definitions of the cities and collecting at https://tomforth.co.uk/humancapital/
Not all of the north is poor. Cheshire is richer than outer London or the home counties. If you adjust for the cost of living more of the north would be.
North poor, south rich is very simplistic. https://en.m.wikipedia.org/wiki/List_of_regions_of_the_Unite...
I think the point is that we would expect East German cities to have reasonable relative growth post-unification because we are taking a relatively structurally deprived area within a wider more successful economy.
That Manchester has kept up with areas where you'd expect to see reasonable growth, is positive. I suppose there's also the hint that this suggests that this has been allowed to happen by central government, but I could be making that up.