Is that definitely the case?

I don't know exactly how well 'sealed' different brand and model toner cartridges might be, but I certainly have taken nearly empty toner cartridges out and tilted, rocked and even firmly tapped them on a desktop surface to redistribute the remaining toner and extract some more page printing. Typically there is minimal or no toner spillage. Depends on the cartridge design.

There may be some laser printers that would be more compatible with being jostled enroute.

Perhaps removing the toner cartridge for travel would allow it to cope better. Putting it back into the original toner box inside a ziplock baggie might be enough to avoid a black toner mess.

What about heat, cold, humidity? How extreme will the environment be for this travel printer?

Will the printer sit unused for long periods of time between travels? Ink jets can dry out and clog the nozzles if not used periodically.