A guess is that Doyle is referring to the fictional character Dugald Dalgetty, who appears in Walter Scott's novel A Legend of Montrose.
I suppose Scott's novel was more widely read at the beginning of the 20th century than nowadays, and in a book like Doyle's the use of a fictional character to describe a historical person was (in a work of of propagandistic journalism) not unheard of. A modern analogy might be to describe (say) Allen Dulles as having had a James Bond-like career, or Admiral Cochrane as having been like Jack Aubrey.
Indirect evidence that Scott's novel was read and admired in the 19th century is the fact that the Governor's mansion in the American state of New Jersey is called Drumthwacket, following the passage in Chapter II of Montrose where Dalgetty introduces himself:
“Truly, my lord,” said the trooper, “my name is Dalgetty—Dugald Dalgetty, Ritt-master Dugald Dalgetty of Drumthwacket, at your honourable service to command. It is a name you may have seen in GALLO BELGICUS, the SWEDISH INTELLIGENCER, or, if you read High Dutch, in the FLIEGENDEN MERCOEUR of Leipsic