St Crispin -
I think FI's point is that this guarantee would apply to all the airports at the London end too, i.e. you should be able to compare the Ryanair STN-Skavsta fare with BA/SK from Heathrow to Arlanda.
That said, I do agree with you about the difficulty of getting refunds, and assume that FR has de-emphasised this "guarantee" since it's no longer a key selling point.
A related refund issue is the PSC which is collected in respect of each departing passenger. If you don't fly, the airline doesn't have to pay the PSC - I recall managing on one occasion to get a refund from Ryanair afterwards (with several letters) but they have now changed their wording to make it clear that all money paid, including passenger charges, is non-refundable. A nice deal, this:
contribution to FR fixed costs if you fly
=ticket price - pax-variable costs
ditto if you no-show = ticket price + PSCs and taxes
Now this may be the same for other airlines, but IMHO the argument is rocky: "don't blame us for these taxes and charges, they're nothing to do with us, we have to pay them to the greedy airports," followed by, "you didn't use your ticket? OK, so we don't have to pay the taxes and charges to the airport for you now, but of course you can't have the money back - it's ours."