If I encountered this syntax in a vacuum, that's not what I would think. I would think that ADD ALL TABLES IN SCHEMA meant add all the tables in the schema to the publication one by one as individual objects, i.e. add the tables that are currently as of this moment in that schema to the publication; and I would think that ADD SCHEMA meant remember that this schema is part of the publication and so whenever tables are created and dropped in that schema (or moved in and out) what is being published is automatically updated.
The analogy here seems to be to GRANT, which actually does support both syntaxes. And if I understand correctly, GRANT ON SCHEMA gives privileges on the schema; whereas GRANT ON ALL TABLES IN SCHEMA modifies each table that is currently in that schema (never mind what happens later).
Yes, except GRANT ON SCHEMA only grants access to the schema - CREATE or USAGE. You cannot write GRANT SELECT ON SCHEMA to grant access to all tables in the schema.