That's nonsense. So long as we're talking about the
mechanics of the idea, which is implicit when talking about a
business plan, then yes, you most certainly
can invalidate an idea. Just because people would like to have something doesn't mean you can either produce that or make money selling it at the price point they'd like to buy it at.
I'd like a hand-cranked machine that takes my daily garbage and spits out Ferraris in under 10 minutes and I'd like to pay $5 for this machine. I'm sure I can demonstrate demand quite easily. Will you fund it?