I wouldn't be as concerned about the bus when arriving, since everyone on the plane, and therefore on the bus, had to test negative before they could board the flight. I know a lot can happen in the five days between testing and flying, but still would feel pretty safe. Getting on the bus to depart might be a little riskier. However, the guidelines do say that less than 15 minutes of exposure isn't likely to cause infection. I don't think the short bus ride is an issue. Just my opinion.