Scam may be too strong a word. It could be that TK are being over-zealous in interpreting the rules: or it could be their interpretation is one enforced by HMRC. Who knows - the Revenue might have got a handle on the amounts of this juicy tax that is escaping them through creative itineraries.
In effect, the application of the rules as described by the OP assumes that the connectivity of the journey (same airline, direct transfer, baggage straight through, boarding cards for both sectors issued in UK etc) adds up to make a powerful case over-riding the use of two tickets to circumvent the regulations.
For APD purposes connected flights are treated as one journey and APD is charged according to a passenger’s final destination irrespective of the number of flights that they take to reach this, provided the flights are connected.
Nothing to lose by asking the Revenue how they interpret the OP's intinerary for ADP purposes.