Can't wrap my head that much about these 2 characters.
Usually anime filler characters and OVA characters are non canon and make things easier for the official, true and only manga timeline.
Though these 2 characters are a different thing.
First of all they're both created by Oda himself.
While Zephyr/Z has never been referenced in the manga, but could be in the future, although it would be quite pointless if not to mention most Marines' sensei, Shiki is different.
First Oda mentioned in the manga that one man was able to escape Impel Down, then, through Episode 0, which is canon, we learn that the man is Shiki, a man that in the Roger era was able to stand against Roger and Whitebeard.
Now, the story of Strong World is non canon, so…where's Shiki after his Impel Down escape?
Obviously such a big pirate that could be Yonkou level couldn't have been overseen by other pirates/Marines but how does he relate to the official timeline?
How do you people explain this stuff?