That's a totally valid and useful way to validate an idea. After a few months of manual labor they will have a good idea of how/what to build and if it is even worth building.
There's nothing fundamentally bad about having Oompa Loompa's behind the scenes, as long as you're honest about the outcomes you can provide.
I agree, though: also a very sensible way to prioritize development work.
Still could be worth doing a bit of manual work like this, but it's worth being cautious about drawing conclusions from it.