The word Shramana is in the taittiriya aranyaka 2.7 and brihadaranyaka upanishad 4.3.22 - so it from the vedic period itself.
Generally, the opposition between a "brahmana" and a "shramana" is - that the brahmana is a gruhasta and in those times mostly a miimaamsaka.
the shramana on the other hand would be a sanyasi. So the "opposites" aspect is based on the fact that one is a house-holder and the other being a sanyasi.
This would be the context under which Patanjali would consider the two words as opposing - or an oxymoron.
Unfortunately, under the 'influence of indologists - the term sramana is routinely interpreted as being only a bouddha (or a jaina) and somehow being
some kind of an opposite to a brahmana. Bouddhas and Jainas were not the only one's who took up sanyas.
Is there anything that i am missing ?
regards
S. Subrahmanya