Some workplaces want you back after a negative test
If you are symptomatic and test negative then of course you go back! For it means something else has caused your symptoms.
If you are required to quarantine (travel) or isolate (close contact confirmed case) then you are not released by a negative test as you could be incubating it (typically 5-6 days, but range is 2-14, and only very, very rarely beyond that)
I suppose it's possible in this case that the traveller had no clue, and had only taken the test because an onwards destination had required one?
(Very glad they were masked, but even so I would not like to have been sitting in the row immediately in front of them!)