It’s impossible to prevent, only reduce.

And that’s what such efforts aim to do. You can’t prevent everything but you can definitely cut down on what is potentially being introduced. This is particularly true when a place is as geographically isolated as Antarctica. For a relevant example I know that if you were to bring a raw stick into Australia it’d be confiscated (or required to be pest treated at your cost) due to biosecurity concerns, and we get literally millions of people visiting per year so that’s a significantly harder containment job than Antarctica would present. Even within Australia there are biosecurity controls disallowing movement of stuff like fruit and grape vines between some of our states/regions.

I would be surprised if biosecurity controls for our parts of Antarctica were not even stricter, given that it is a largely untouched landscape and reducing impact on it is considered worthwhile to do these days (not so much in the early days of the Antarctic program, but we try to do better now).