It does take substantially longer to get to Mars than to get to any isolated pockets on Earth. So unless the pandemic’s incubation period is longer than the journey to Mars, it’s likely that Martians would know that passengers aboard the ship were infected before it arrived.
The absolute travel time matters less for disease spread in this case. It doesn’t matter how long it would theoretically take to travel to North Sentinel Island if nobody is actually going there years on end. Disease won’t spread to those places naturally.
And if an organization is so hell-bent on destroying humanity as to track down every last isolated pocket of human settlements on Earth (a difficult task in itself as they’re obscure almost by definition) and plant the virus there, they’ll most certainly have no trouble bringing it to Mars either.
I’d worry that if we’re looking at a potentially civilization-ending pandemic, a would-be warlord with a handful of followers decides that north sentinel island seems a kind of attractive place to go all of a sudden.
I had always assumed that any organization trying to destroy the world with an engineered pathogen would basically release whatever they made and then hope it did its work.
IDK, this topic gets into a lot of information hazard, where I don’t really want to speculate because I don’t want to spread ideas for how to make the world a lot worse.