on the Q Board, as I understand it, the surfaces are waterproof, the joints between would need the appropriate waterproof sealer. it's designed to be laid side by side and just a small seam is "exposed"
I think the issue is the sides, that thickness is not designed to be exposed or not sealed. I believe it is a bit porous. So this is where I would take extra care. Also the orange/pink center is definitely not UV resistant.
Personally, I would go with a traditional hardi-backer, which is cement based, and normally, as was asked, has fiberglas mat embedded. I have used the higher end, waterproof stuff as an untreated, unsealed base for my switchyard, it's about 10 years old, looks new, and I clean the switchyard with a hose or pressure washer.
Greg
The picture below is before I used plastic wood to space it from the ground, but this picture shows the product off better. It looks the same 10 years later.