The LATCH strap and the seatbelt are each designed to stretch up to 20% in a collision. Same with the harness, but that has no bearing in this conversation. That increases the "ride down time", how long it takes for the stop to happen. You don't want to stop immediately. You want to slow it down. It's like slamming into a brick wall versus a padded wall. The padding slows you. You'd much rather hit that than straight brick.
Ok, back to why. Each of those are designed to take the crash energy and stretch with them. If they're both in use then NEITHER of them may stretch properly. Back to baseball, have you ever seen two people chasing the same ball? "I got it!" "I got it!" Neither ends up with it. However, the energy must go someplace as it can not simply be destroyed there, and the next place it'll go is into the carseat. Well, the carseat may absorb it without a problem. It may not be able to absorb it and it may crack in half, or it may simply transfer that force to the next thing in line, which would be your child. So instead of hitting a nice padded wall your child has been slammed chest first into a brick wall. One that unfortunately will allow the head to slam right over the top of it. So all of that energy goes into the weakest point, which is the neck.
So instead of the LATCH or seatbelt stretching you may end up with your children's neck stretching. With their skeleton not yet fully ossified that may result in either a broken neck, or something called internal decapitation, where their head comes off of their spine, but it's not removed from their body. In lucky cases that results in paralysis. In EXTREMELY lucky cases it results in nothing more than a couple of months of recovery and rehab (this just happened to a four year old I know of on my local mommies board, she was in a booster and "dislocated" her spine). In most cases it results in death.
OR, if the carseat cracks you now have that behind your child, riding down the forces on two pieces. At the very least I would expect severe bruising and cuts from the plastic on the child's back. In the worst I would expect a broken back.
All of this can be prevented by simply using the seatbelt (continue to use the top tether) and using the seat as the manufacturer has tested it.
Your husband can also feel free to call Britax and chat it over with them.
Maybe he'll find out the actual reason why. But these are our theories and concerns about it.
Wendy