Your semantic differentiation makes no sense - if “airport prices” are lower than high street prices by exactly the amount of VAT (20%), then how are they NOT VAT free?
Also, it’s NOT 20% off the price after VAT has been added. Instead, it is the base price without VAT added. For example, say the base price is 5000. Add 20% VAT = 6000. But a 20% discount of 6000 = 4800. A lot of people get confused by this and say that it isn’t VAT free because the effective discount on the base + VAT is < 20%.
For all the watches I saw at the T5 Watches of Switzerland store there were two prices - the retail in the UK including VAT, and the price without VAT. As an international traveler, I only paid the price without VAT.
Ultimately, as others have said, if you know what the price of a watch is in your area (where ever that is, and accounting for local discounts and taxes), and you are aware of exchange rates, what your credit card fees are, and import duties, buying duty free MAY be cheaper. But it may not. You have to work out the math yourself, and it will change from case to case. Don’t just assume it will be cheaper.