That does not sound right to me, makes no sense in terms of typical car seat installation design. Seats are designed to be installed in a matter of seconds in the factory and in factories the interior work happens after the major exterior body work is completed. They don't want to risk damaging the interior or getting it dirty so that is why it happens just before the vehicle goes out the door. The seats will be one of the last things installed on the vehicle. I am a former factory girl, aircarft not cars, did a few years in final assembly. But I have watched videos of the assembly line processes on cars such as Mercedes and Volvos. Anyway what that person told you make no sense at all in terms of how vehicles are assembled. But standard methods sometimes have exceptions...
Just get a socket of the right size and try removing them. If they come out then the is problem solved. I don't have that particular van. But I have never had trouble removing the bolts on seats in various vehicles using a wrench or a socket, whichever is easier to fit in the space. If it won't budge and it looks rusty then you might need to use some rust busting liquid and let it soak in overnight before you try again.
