yup they need the mobile app specifically because there was trade frauds happened before, with cs items that costs hundreds. They can't just go with using a normal TOTP because the mobile app is also a 2 factor approval for every single trades you are attempting to make.

I think you can go without mobile app and use only e-mails, but it will cause the items to be held for days which people who trade frequently will not want to trade with you as their inventory moves fast.