3 interesting takeaways:

1) Infection with SARS-CoV-2 induces virus-specific T cells.

2) Patients recovered from SARS 17 years ago still possess virus-specific memory T cells displaying cross-reactivity to SARS-CoV-2.

3) Over 50% of donors with no infection or contact with SARS-CoV-1/2 harbor expandable T cells cross-reactive to SARS-CoV-2 likely induced by contact or infection with other coronavirus strains.

This doesn't mean having T cells will make you immune to Covid. We just don't know that yet. However, this information COULD explain why ~50% are asymptomatic.