If a country choose not to shut its airports doesn't mean your country has to accept incoming flights. If a country thinks people are coming in with a pandemic disease and don't shut the entry points (ports/airports) that's on that country's leadership.
I've heard this multiple times as a reasonable possibility, and yet it seems absurd if you think about it: are you trying to accuse someone of leaking into the environment a virus that actually comes from the environment? And how do you even prove something like that?