There's some pretty questionable stuff going on in that article.
First, it's highly specific to Australia, where the impact of Eurasian cereal farming is profoundly negative, because they're nothing like the native grasses. That could probably be mitigated by not growing Eurasian grains in giant monoculture farms, in favor of edible native crops. There are something like 6,000 native plants in Australia that are edible by humans; very few of them are farmed commercially or have even been thoroughly investigated for that purpose.
Also, it then goes on to say "If more Australians want their nutritional needs to be met by plants, our arable land will need to be even more intensely farmed." This is a very questionable assumption. Generally switching from commercially-raised meat to a plant-based diet results in a lower food-crop consumption profile. This is because meat animals are frequently raised, or at least fattened during the terminal months of their lives, on human-edible crops (in the US it's almost solely corn).
Anyway, I get the impression that article is someone playing devil's advocate / provocateur, which is not something I have a lot of time for. There are lots of solid, peer-reviewed articles [1] investigating the ecological impact of plant vs. meat-based diets; there is a wide consensus that plant-based diets are significantly more sustainable. There are probably places on earth where that's not true, and perhaps Australia is one of them, but it's not broadly true for most of the planet.
[1]: https://academic.oup.com/ajcn/article/78/3/660S/4690010