Most common law jurisdictions give trademarks as a matter of law just for using something in commerce. I think Vanced could have legitimately sued to stop another product called "Vanced" from using that name. What "obvious reason" would prevent them from having had a trademark?

Copyright infringement is a much stronger claim at this point if they were actually upset. Vanced obviously had original code, so to the extent ReVanced copies that without permission, it would be infringing.