That's my job right there (I sell alcohol to duty-free operators), so I have a little insight.
Let's work first on the Duty-Free concept. What makes the products duty-free is not that you can buy 1 litre, 2, 6. It's that they were never taxed by the government. In the case of locally-made whisky for instance, no excise, not VAT, nothing. Straight from the distillers to a bonded warehouse, and onwards to the DF shops. And for imported products, say a French Cognac, or a US bourbon, no import tax, excise, VAT. Straight from the ship to the same bonded warehouse. In either case, the products never hit the domestic market, and are not fit to be sold locally. Which is why they mention For all passengers travelling OUTSIDE the UK.
The mention
Staff confirmed that the duty-free limit for travelling outside of the UK is 1 litre.
is incorrect. You can take as many litres as you want. The one-litre duty-free limit is for import into Ireland (if you're flying to another country, the limit could be different).
So indeed, you could buy a full suitcase worth of booze, as I've seen people do (why do you think in many airports liquor DF stores give away suitcases?), and they're all duty-free. At the point of purchase. It's duty-free for export.
Now, when you land somewhere else, a new set of problems crop up. The duty-free allowance for import. Different floor, arrivals vs departures, different rules. That country's Customs service allows incoming passengers to bring in a certain amount of alcohol, tobacco, perfume, etc, without having to pay tax on them, and if they exceed that amount they will have to pay tax on the remainder. Fair enough.
Some people may gamble, and hope they won't get caught at their destination. In which case good deal, they got booze at a cheaper cost. That two-bottle deal worked for them. In other cases, they could be travelling with Da Missus, who's buying perfume next door. John buys a two-litre pack, and if stopped by Customs on arrival, points at Jane. Two of us, Guv, two bottles.
And finally, there's the case where John fully knows he bought 1 bottle over the limit, and intends to abide by the law. He goes to the "Goods to declare" lane, to pay the tax on the 2nd bottle, of his own volition. He might still get a good deal, in the sense that the cost of the 2 bottles, one untaxed, one taxed, is lower than buying 2 bottles at destination (especially in countries with heavy taxes on booze). This works in countries that don't tax everything if you exceed the DF allowance, as mentioned by @Jack below in the comments.
The fact is, "duty-free" means that no government levied taxes on the products in the country of purchase. Past tense. It's not a promise another government (or the same government if you bring the alcohol back) won't tax you when crossing a border.
