In his introduction, Walt Disney states that Dr. Syn/The Scarecrow was a historical figure. Syn was, in fact, born in the mind of novelist Russell Thorndike as the hero of a book series first published in 1915.
At the very end, Dr. Syn and Sir Thomas are talking of Harry's rescue. Sitting a bit further away are Kate and Phillip Brackenbury. Dr. Syn mentions to Sir Thomas that the Scarecrow was able to free Harry where Dr. Syn could not. But Phillip was there when Dr. Syn entered the dungeon as himself, dressed as the Petty Officer from the naval picket. Phillip knew it was Dr. Syn who helped the prisoners escape. If he overheard Dr. Syn, it should have told him just who the "Scarecrow" really was.
The American character Simon Bates speaks with a modern American accent, while all of the English characters speak with modern English accents. At the time this story is set (1775), the English spoke with an accent much closer to the American accent. The modern English accent wasn't developed until the 1800s, and was first created by wealthy southern Britons as a means to differentiate their speech from the lower classes.