I hear that too. Is there any info about why far apart spacing is bad/dangerous. Just curious. I’ll try the center seatbelt and hopefully I can get it as snug as the latch sustem.
I am not entirely sure, but I suspect it has something to do with the angle of the strap being too far from perpendicular to the LATCH receptacle. This is not a Tesla-only issue. I have seen the manuals for several other cars, including my wife's Volvo suv, warn against doing the same thing.
I used the center belt on my Model S for a while when my son was still rear facing but over the LATCH weight limit specified by his car seat. It is certainly more of a pain than using the LATCH system, but it is possible to get it just as tight. My advice is to push down on the car seat as hard as possible while you tighten the belt.