Well... here's my take:
Spoiler:
You skipped the credits didn't you :P
Spoiler:
But is that extra scene canon or not?
The way I see it is that Booker, Comstock, Elizabeth and the Lutecs and Lady Comstock exist across ALL the universes (the last two are stated in game). So what happens during the ending, does it affect every universe or just that one?
Think to when Comstock eventually dies and what happens then
I tend to take it as canon, Spoiler: