The knuckle-dragger would have only looked at the outside edge and your pic shows the wear marker on the right looking deeper than the inside one so I’m guessing the tyre has worn differently over it’s width so it could look fine from the outside.
Not making excuses for them but if you’re not happy with the vehicle presented to you do not take it. I regularly rent cars and vans via work and the amount of times I’ve had to reject a vehicle is astounding considering we use the big companies. Bald tyres, knackered wipers, headlights not working etc are all common.
Oh and do not drive it now you know the defect is there. It is your responsibility to ensure the vehicle is roadworthy so you get the fine, points or worse.
EDIT: Vitos are under 3.5T so the limit will be 1.6mm. It’s only 1mm for vehicles that are over 3.5T and are limited to 56mph. The only exception is coaches.