Buddhism generally is just a "be gay idc" religion from what I know

It is 80,000 years ago

https://rockartaustralia.org.au/australian-dig-finds-evidence-aboriginal-habitation-80000-years-ago/

And no, they aren't a single nation, that's why each group has claim to their specific land, not any more nor less.

And, the Aboriginal Australian version of possession of land is different than the European version, the land is not "owned" but rather it is tied to a people through religion, culture, hunting, etc.

Please learn something about Australia before making ridiculous comments

If you want to get into the semantics of it, we can:

Always has always had a range of meanings, the most common being "forever" or "every single time"

Another common usage of it is using it to exaggerate sentences.

In this context, it is using definition number 2

Always has been their land means "it has been there land [for a very long time]"

Just like how European land is considered "always European", Aboriginal Australian land is considered "always Aboriginal Australian"

Another reason people say that is the Aboriginal Peoples of Australia have a unique connection to the land, more so than other cultures do to the land they are on

Their religion, lifestyle, presence, culture is uniquely tied directly to the land that they claim.

I'd suggest speaking to an Aboriginal Elder for a better explanation as I'm merely an Australian who only knows the gist of things

It has been their land for 80,000 years.

And still is, aboriginal peoples have their land recognised here, its a pretty common thing and this isn't unusual