Signing an NDA before the interviews is a very common practice (unfortunately)
Or, to put it another way, this is not a large-scale sustainable enterprise for a startup; even if it "succeeded", the business environment will adapt to eliminate it. Interviews are too expensive to let companies be screwing around getting paid to pitch crap at you in an "interview".
Well, in this case, probably exactly what is in this article?
Seems weird to let leak “secrets “ during an interview that require a NDA though. Just be vague and tell them the secrets later.