This really is most odd. If this is a BA codeshare with BMI, then it is unique to Leeds-Heathrow and doesn't seem to be on any other bmi route like Heathrow-TeesSide, Heathrow-Jersey or others which you would think would also be included if they are doing a deal.
However, you can't deny that Leeds is on the BA route map with a link to Heathrow; and it's also included in the drop-down list of destinations in the BA booking engine, when they haven't had any service (even a codeshare) out of Leeds since BRAL pulled out several years ago.