Café de Nuit

Where does Fair Barovia from Dungeon #207 fit in the timeline ?

It refers to a story set centuries ago (Léo Dilisnya hiding under a false name in a monastery).

Time line, year 398 : Strahd hunts down and punishes the traitor, Leo Dilisnya; learns of the monastery on Mt. Baratok. (This same monastery may be the one later used by the Guardians and which ends up in Markovia.) [DoD pg. 16; I,S]

Actually, Fair Barovia explicitly mentions Strahd XI as the current ruler. The events of Leo's original capture take place quite a long time in the past.

EDIT: It's worth noting, however, that the adventure also says that Leo was entombed for just fifty years before escaping, which would conflict with the rest of the history they give in the adventure (since they accurately recount the same events as described in I, Strahd). The simplest "fix" would be to say that Leo was buried for hundreds of years, but was kept in some sort of stasis by the Dark Powers, either knowing the players would get involved eventually or as a recurring punishment for Strahd, making him deal with Leo over and over again.