Fowler knew nothing about Canadian English in the 21st century. His guide was written 90 years ago in Britain. "An historic" may have been appropriate in Britain at the time, along with "an hippopotamus", "an hotel", "an hysterectomy", and "an hilarious joke". Those may all still be correct in certain dialects like modern Scottish. None of them are correct in Canadian English.
If the H is silent, it gets "an". If not it gets "a". It's that simple.
"I'll be back in an hour." Yes.
"I stated in an hotel last week. It was an historic hotel." Nope.
I don't know what it is about the word "historic" that has people wanting to put "an" in front of it. But I'm glad the Bank of Canada got it right.