I've doubted this tale for some time, but hadn't come across any conclusive evidence. I won't get your hopes up. I still haven't. But I have found a couple of quotes that at least cast doubt upon the theory.
Hop-growing has been concentrated in a few regions of the British Isles for centuries. They weren't grown to any noticeable extent in the North of England, Scotland, Wales, the Southwest of England or Ireland. Though plenty of brewing went on in all those regions. Guinness is a good example of a brewery than brewed beers using plenty of hops, while situated far from the nearest hop field.
Consider this: hops aren't exactly a heavy cargo compared to, say, barley or barrels of beer. And there was already a large export trade in beer in the 18th century. Initially mostly Porter issuing from London. But by 1800, Edinburgh brewers were sending beer in the opposite direction:
"As we should expect, these architectural additions had their counterpart in the growth of his [William Younger II] sales. These had been making such phenomenal progress that even in London, where competition was fierce, he was well-established by 1830, thanks not only to his own excellent product but to the fleet of fast Leith smacks, which, in half the time taken by his father's old brig the William were conveying his hogsheads south and bringing back in return hops from Robert Tooth of Cranbrook and other Kent growers."
"The Younger Centuries" by David Keir, 1951, page 39.
It seems only logical that, as Scottish brewers had a liking for Kent hops, that these were carried on the return journey back to Scotland.
On to my second piece of evidence. Quite agood one, as it talks specifically about the flavour and hopping of Edinburgh Ale:
"From this pernicious though ingenious manufactory [distilling] willingly turn to one of a more advantageous nature, which for the welfare of the community, it were much to be wished could supersede the former; that is to say, the trade of brewing ale, which has of late years been carried to great perfection in Edinburgh. Formerly a brewer, who had established his works in the southern district at the Pleasance, Mr Bell, was more celebrated than any other in Scotland for the preparation of malt liquor ; but his ale had the fault of being extremely intoxicating. Mr Giles of Leith afterwards acquired great reputation for preparing ale of uncommon beauty, capable of being preserved for a long period. It is understood, however, to be chiefly acceptable to persons of a peculiar taste, on account of its bitterness, arising it is supposed from the large quantity and strong boiling of the hops used in its preparation. But the ale which has acquired the highest reputation, and is now bought up with great avidity in London and other distant markets, is that prepared by two brothers who carry on business separately, Messrs Younger. When properly managed, this ale is as transparent as Sherry, without froth or sediment, and of such a moderate degree of astringency or bitterness as to be universally acceptable. It were well that, in consequence of its growing celebrity and popularity, it could find its way into general use among the lower class of people to the exclusion of ardent spirits."
"The Beauties of Scotland Vol I", by Robert Forsyth, 1805, pages 159-160.
This passage describes two very different types of Edinburgh Ale. One, brewed by Mr Giles of Leith, was extremely bitter. The other, brewed by the Younger brothers, was more mellow and less bitter.
Looks like the reality was a good deal more complicated than the tale. But isn't that always the case?