Ah......the old Boro classic of using the population of the town of Middlesborough to justify their attendance, ignoring Stockton, Hartlepool, Darlington, Redcar and the rest of the Tees Valley. All of these have a Boro support well in the majority.
Tees Valley population in the 2021 Census is 678,200. That's not counting North Yorkshire.
Population of Sunderland is 288,000 including Washington, Hetton, Houghton. Yes, we draw from other areas but not huge percentages from areas with a combined population of 400,000 higher.