As the article states, it is technically a gain of function experiment. But what is actually prohibited are dangerous gain of function experiments. This specif

As the article states, it is technically a gain of function experiment. But what is actually prohibited are dangerous gain of function experiments. This specific use of antibiotics resistance is also extremely common. Pretty much every single microbiology experiment starts this way, and every time you want to genetically modify bacteria you do this. This includes every time you want to produce any protein, because you need genetically modified bacteria for that. This is a large part of all labwork in this field. As the article notes, the antiobiotic used is one that isn't used for humans. So there is not significant additional danger due to that modification. The bacterium itself is much more dangerous than most that are handled in the lab, but that's why they're in a BSL-3 lab.