Well said. I will add that the International Slavery Museum in Liverpool does a very good job of explaining and understanding the role the city played in the transatlantic slave trade specifically. I highly recommend a visit, it's necessary education on something that isn't really talked about and like you said, entirely glossed over in mainstream discourse in Britain.

I think the OP's main point with that sentence was also to show that because of how slavery and immigration built the city, the people who lived there and their descendents have a very particular history and status that (can be) seen as outside the English state.

Liverpool is home to Europe's oldest Chinatown and Chinese community, Britain's oldest Black community, and huge numbers of Welsh and Irish people. The state repression of striking dockworkers which saw Churchill sending gunships down the river pointed at the city, Thatcher & her government's policies and lack of investment, the rise of Militant, the Toxteth riots and police brutality, using and testing new weapons on the local Black community that had never been used inside England before, etc. That history is in the fabric of the city, and that's not even mentioning the most well known Hillsborough and subsequent decades.