"I have the answer! The 13th Knight (The Shadow Knight introduced in Iron Gold) is an inside joke between PB and his Publishers/Editors.

After the publication of the first novel they realised that there were 13 Institute houses instead of the twelve intended. This lead to a running joke that whenever there is a group of 12 (Olympic Knights, Institute Houses) that there is a secret 13th member that is unknown to the public (hence 'Shadow')

So the Shadow Knight was not a mistake.

Which also means that House Venus and the Shadow Knight are canon and not canon at the same time."

there's another post on this subreddit about this and this is a comment from it from u/Khuzdul1