If they sold that box with a DTL topper it would probably be quite popular.
Although some people, maybe new users who may just pick one up on the spur of the moment, may not even know what they are buying.
Having said that, squonk boxes only house one battery and they get used for DTL. Tube mods too, people use them for sub ohming as well.
I personally wouldn't buy it. I use "dual" box's for the prolonged power they provide, and i'm a MTL vaper !. So for 30 quid, or even if it was a tenner, it would be any use to me. Plus i hate the fact that at some point in the not too distant future i'd be throwing it out when the built in battery can no longer hold a decent charge. Starts to get a bit annoying even when a battery like that loses only 25% of it's capacity.
Have you sold many ?. Who to ?.