Very simple. NT's memo states that Baby is making a positive cash contribution. What this means is that it is generating plenty of cash from advance bookings, which is not surprising given its high number of sunshine routes where you get advance bookings for the summer season. Generating cash does not mean that the airline is profitable - far from it. You could still lose a significant amount - let's say £20m - but on a turnover of six times that, be cash generative during the January/February/March period as the bookings come in for the summer.
On the other hand, bmi mainline could be cash neutral or negative right now - given later-booking business travel routes versus day-to-day losses - but be more profitable (or less loss-making) than its subsidiary.
Don't get cash and profit mixed up.
I don't doubt that bmi baby's costbase is in better form than mainline, and you are right in that the weight at the top of the seniority list in mainline is one of the key issues that the airline has. How you then choose, as a management team, to deploy the cost-bases to revenue earning activities is then a totally different matter.