Auto Express seems to be the source of those figures, which show that diesel cars as a class have (slightly) higher average mileage than Teslas, and roughly 33% higher average mileage than full electrics as a class.

Again, it seems like selection bias, in that those high-mileage drivers who can use electric do, and those that can't buy diesel. Presumably there is a cohort of diesel drivers who would switch to electric if they offered sufficient range or faster charging, especially as diesels outnumber electrics ~2.5:1 in the UK [2].

[1] https://www.autoexpress.co.uk/tesla/352144/tesla-drivers-cov... [2] https://www.statista.com/statistics/299031/fuel-types-of-new...