This query arises due to the concept that a person attains the status of 'Brahmin' due to his birth. This particular concept came only later in India. Bhagavad Gita clearly disregards this 'birth' right.
A person becomes a 'Brahmin' by his Karma and not birth. Anybody can attain the status of a brahmin, irrespective of his birth.
So, Devavrata was a 'Brahmin' due to his Karma i.e. Knowledge. He was the most knowledgeable man of that particular era. Lord Parashurama taught a boy who attained the status of a 'Brahmin' via his 'Karma'.
- Sage Vasishtha, one of Devavrata's preceptors himself wasn't a brahmin by birth but Karma.