You're right. 5kg is next to nothing. I suspect the makers are being super cautious on account of the quick release catch.
However, it copes perfectly well with that 30litre stuffsack containing, among other things, tent, groundsheet, peg-bag and poles, thermarest-a-like, flip-flops, waterproof and assorted junk that doesn't travel anywhere else. In short, quite a lot more than 5kg.
I'm not prepared to destruction test it to discover it's limits, though.