It also says (specs on Home Depot web site) not listed for fixture support in ceilings, so it isn't even a question of 5.6 < 6, but rather "not designed for this purpose at all".
I believe the problem is not "plastic" but rather the wimpy little "swing clamps" - by design then can't support much.
The solution is a box that is actually nailed or screwed into supports. There are plenty of options like: For a fan, you really need to make sure it is securely attached to a support (e.g., stud). For a light, there is a bit more leeway as it is (usually) not as heavy and does not have the constant motion/vibration of a fan.
With a horizontal fixture you should be fine. I use old work all the time for vanity lights, the one case where I would use caution if the light extends away from the wall more than a few inches, I have mounted carriage lights with old work plastic boxes but if the fixture extends further than that I would be moving to A metal old work box.