It is revealed here that witches can block fairy magic.
If Alden Bitterroot is indeed an ancestor of Denzel Crocker, that would mean Crocker has witch blood in him. It is unknown if he has "witch powers" like Bitterroot as he's never demonstrated them in the show.
Dale Dimm is apparently thought by many people to be a myth since there was apparently no actual proof he existed. This was changed when Timmy was able to take a picture of Dale Dimm and the rest of the Colonial people together holding a sign that welcomes people to Dimmsdale. This picture was then show to A.J. as shown in Timmy's float.
This episode (along with its partner episode, Kung Timmy) rarely air in reruns for unknown reasons.
Apparently people in the present have always believed that Alden Bitteroot was a big muscular Pilgrim that got rid of actual witches and didn't realize he was just a Pilgrim version of Mr. Crocker until Timmy found out about that when he went back to when Dimmsdale was found.
The title of this episode is the exact same title of an episode of Scooby-Doo, Where Are You? Coincidentally, both involve witches.
Salem Witch Trials - The episode's plot is based on the Salem Witch Trails from 1962 to 1963; which was the chase of people supposedly being involved in witchcraft.
Dimmsdale is in California, which wasn't settled until sometime in the 1700s. Also, during the year that Timmy and his godparents travel to (1665), California was under the control of the Spanish (from Spain) rather than the British (from England).
Also, California didn't become an official state until September 9th of 1850, and became a state largely because of the California Gold Rush (which happened from 1848 to 1855).