Scion exists to handle unexpected variables along the lines of weapons the people might produce that somehow get past the thinker, shard interactions that scale beyond the test, or... aliens from another world. Yeah, Scion exists specifically to combat and manage forces like Superman. In the medium and even the long term, dropping a Superman into the picture means that Scion has a reason to exist, one that goes to his core functioning, core purpose, where the lack thereof is his biggest weakness. Superman as an unknown variable is a valuable variable, and any attempts to get home may be forestalled - Tinker makes a gate, uses Superman's signatures or whatever to key a way home, and Scion shows up to Still it.
