Thomas Cook is an integrated organisation, of airline and holiday resort provision. How they divide up the revenue from an overall holiday between the different areas of the business is up to them. If they are minded to sell the airline part it looks good to divert the profitability in the accounts to that division.
Varying the dividing point between the different divisions of such an integrated business is not new. Back in the 1970s/80s when BA owned airline British Airtours, and tour operators Enterprise and Sovereign, all the margin was credited to the airline side, because the airline-oriented management did not want to give discounts to their own tour operators.
The principal asset of the airline side is the assured business from the holidays. If they sell off the airline that assurance is no longer there, and the continuing holiday operator would have no reason not to buy their airline capacity on the open market. If the airline side is so profitable then their pricing is presumably not the lowest.
Comments that the holiday airlines are a declining market ignore the likes of Jet2, who have made substantial, and financially worthwhile, expansion into exactly this market in recent years, doubtless having scooped up a lot of Thomas Cook's potential along the way. Just a different management style. People have not stopped going to the Mediterranean and elsewhere, far from it.