Well, that's not strictly true. Doing things that don't scale (a prime target for startups) tends to include things you are just pretending is software, but you actually do by hand.
It's entirely possible to have customers before implementing the software.
I'm not talking about a landing page. I'm talking abouy something that looks automated but is really powered by people on the backend and would never scale without actual software powering the product.
It's pretty rare, but it is a thing.
Take for example TillerHQ (never used an, but it seems like a genius product). You could MVP it as a form the collects bank login info and then has a person manually copy the bank statements into a spreadsheet for the customer. Highly dicey due to privacy concerns, but it illustrates my point perfectly.
It's entirely possible to have customers before implementing the software.