I flew this route in mid Sept for a long weekend, and the load factors mentioned seem to correlate with my experience.
As far as yield is concerned, all I would say is that there were a lot of 'suits' on the Monday rotation ex BRU, and some of them seemed to be carrying LH boarding passes. As with the flyBMI service previously, those passengers were very familiar with the 'leave your larger hand luggage at the foot of the aircraft steps' routine.
Obviously in peak summer periods, airlines need to make hay while (hopefully) the sun shines, so they will be looking to optimise load factors & yield. Business travel usually tails off in July/August, so it's not a surprise that SN may feel they can get more bang for their buck by deploying the aircraft elsewhere - or CityJet (who do a lot of work for SK) may have other uses for the aircraft, and that has been factored in to their agreement with SN.
At least from a BRS perspective the LH service will still be operating to mop up *A connecting traffic - albeit probably at a price premium!