Looks like they've not updated that site for a while considering 1 bmi mile = 1 Avios.

You can only ever estimate the cost of an airline mile, it has no fixed value unlike if you were to use your clubcard points at Tesco (yes, you can do that).

People more knowledgeable than Martin Lewis (unless Raffles is Martin Lewis!) would have probably valued diamond club miles more than that when bmi was still a separate entity.