Is it a code violation to use seal tight to cross an expansion joint in a parking garage instead of using expansion couplings

Using flexible conduit at an expansion joint has been done and is allowed by Code.

What article or section in the code book says that you can use flexible conduit to cross expansion joints in parking garage

If the flexible conduit you are referring to is LFMC (liquidtight flexible metal conduit) see article 350.10 for uses permitted and 350.12 for uses not permitted...
My opinion would be that this is permitted but you know the specific conditions so you should read this section also...