Maybe it is because I'm older, but whenever I see this advice to build a fake landing page and see if people sign up... I recall that selling vaporware was reviled in the 90s, but somehow now it is considered a wise business tool. It still feels scammy to me. If you have a market in mind, but don't have people in that market to talk to directly to validate your idea, you are aiming at the wrong market. Otherwise all these techniques prove is that you can collect emails, not that those emails will actually become paying customers.