I also don't believe it. In fact, I'd feel duped if I sign up with my email only to find out the product doesn't exist and the owner was simply soliciting interest. It's fine if you say upfront the product doesn't exist and offer users the option to register interest. It's not fine if you hide the fact the product doesn't exist. It's disappointing that so many programmers think this is ok.

Here's an example of someone who created a UI concept that suddenly went viral, but which ultimately never got built. There was never any deception - everyone knew it was a concept piece but many expressed an interest seeing it built:

https://www.vanschneider.com/story-dotmail