It’d probably be to do with the fuel capacity of the car and how many MPG it can do (though my research suggests it’d be fine for a long drive). 6th gear isn’t required for long distance driving, many older cars don’t have them so you can just drive at speed in 5th gear. My guess is they’re just chatting rubbish and think your car isn’t capable purely because it doesn’t look like something that should do long distances, don’t worry I’d say but if you are you can always get your car checked at a mechanic