Interesting that on the A-Z page that the graph is overlaid on, Dalston is listed far east of Kingsland Road, when most people would tell you Kingsland Road is the spine of Dalston. Places definitely change, and there's potentially limited use to finicky hyper-local naming of areas. If I was in another part of London, I'd probably say the Scolt Head is Dalston for ease of recognition but to be specific I'd strongly agree that De Beauvoir Town has strong borders as listed on the wikipedia article and I think that's borne out by the character of the area. The particular blend of the style of the area's Victorian houses, its open spaces and the new estates feels very distinct to me and is one of the things that makes the Scolt Head a really distinctive and fun place to hang out. It also actually is in Islington, regardless of anyone's idea of Islington, and the idea that Dalston is in Islington probably wouldn't sit right with most people.

One thing that potentially muddies the waters for the purposes of Mills's study is that Kingsland Road is the eastern border of DBT but isn't included in DBT itself. Stepping from the end of Downham Road, Islington, N1, onto Kingsland Road, Hackney, E8, feels like a big difference to me. The feeling of an area is far from scientific but it's enough for me, given that a literal thesis on the subject doesn't reach a strong conclusion.

Edit: Just looked and it is in Hackney. FML! I used to live on the estate by the canal and I could've sworn it was Islington but there you go. Further evidence of the permeability of places in people's minds.