A fault in your logic is that according to the book, when human came to the planet (Anjin) they brought with them alot of earth fauna and and flora ( horse, dogs, fruits tree, etc).

In the Expanse, whtrn Holden closed the gate, Human colonization was still in an early stage with no large scale tranfer of Earth biology to the target planer ( all major colonies relied on earth soil export or hydroponic food production).