Basar is not d’Oraisa. Please find me a source that says it is an issur d’Oraisa for a woman to walk around in public with uncovered arms.
This is what R’Moshe says: hair in a vacuum is not erva b’etzem. It can become erva, however, if it is always kept covered. Basar is erva b’etzem but it can become not erva if it is always kept uncovered (which is why a face is not erva).
Now, because there is a chiyuv min HaTorah for married women to cover their hair, m’meila the hair of married women will become erva and it will be assur to daven in front of the uncovered hair of a married woman.
You are mixing up the cause and the effect.