Language is fluid, or we would all be speaking some variant of old German on this forum (try reading Beowulf in the original).
It's not grammar, it's semantics.
Words are social. When one person tries to rewrite the meaning of a word, unless they have significant social clout they are just going to be misunderstood. When many people rewrite the meaning of a word, they change the meaning of that word.
But using the phrase “biological gender” isn't using them in the old way: the way in which they were synonymous is in reference to a idea which conflated sex, gender identity, and socially ascribed gender. If one wants to refer to that meaning, there's really no reason to prefer one or the other, though either will be unclear to many modern audiences without additional explanation referencing the outmoded concept being invoked, because the concept being referred to has lost currency.
But if one is discussing the separated concepts, then one cannot honestly use “sex” for the sociological or psychological components or “gender” for the biological component and say it is the “old way”, as the old way doesn't recognize the there being separate components.
You just contradicted yourself. If the term 'gender' conflates several ideas, and adding the term 'biological' distinguishes them, then the previous poster hasn't changed the meaning of the word 'gender'. He's incorporated the new understanding but adapted the older language. Either way, the mere fact that nearly every person reading this understood his meaning shows that his expression was adequate to express his meaning.
It doesn't. It, when used in the old sense for which sex was equivalently used, refers to one idea which does not recognize the physical, psychological, and social elements as distinct. It does not conflate different ideas, it predates the idea of a distinction; the idea of the distinction is concurrent with the terminology which incorporates it.
“Biological gender” is neither the new common usage (which labels the biological element “sex") nor the old usage (which refers to an indivisible trait.)