J Bar roof mounts

The instructions on my J bar mounts say to strap kayak only to J bars not the lumber rack.  What say the experts?

I would follow their recommendation.

Do you have a picture of the actually model?

Here is a pic.

I believe so there's no wiggle room for the kayak to jump out of its position during transport is why they're recommending to J bar.

Just seems like it would be more secure to strap directly to the cross bar not the J mount.

Just seems like it would be more secure to strap directly to the cross bar not the J mount.

They're concerned you'll create a moment arm and cantilever the assembly if strapped 2 the underlying rax.  I have thousands of miles with my J mounts, and they are not very stable. 
Research here and you'll read a few failure modes.  High convenience, only moderate reliability