The advice makes sense, but isnt it a bit of chicken-and-egg? How do you sell something you haven't built? (or is it really .. build a little, or enough to illustrate; then 'sell' to validate and seek market fit; and then finally truly build?)
"Sales" is a long term process, but the first meeting usually starts with understanding the customer's problem. If you build something before that (and demo it at the beginning of the meeting), you can bias the process with your assumptions. Instead, if you try to get inside the mind of your customer, you can get closer to the correct solution.
For a lot of those first sales meetings, you don't necessarily need technology to demo. Instead of showing/telling your customer, you're starting by listening. Then, you can build a small demo of something interesting before the next meeting (often, this is more to keep the conversation going and explore new ideas than to immediately solve the customer's entire problem).
I agree with the other comments mentioning an initial (days/weeks long) MVP build cycle. However, over the past few years, I've found it helpful to adopt this mindset: "my idea for the best possible product is at least slightly wrong, and the more code I write the harder it is to fix". There's a psychological concept of functional fixedness, where you view an object only as its traditional use. I think a similar concept exists in technology. Particularly as a founders-only company, the technology you build has some inertia, and impacts your mental models of the problem you are trying to solve. The more code you write, the higher the "cost" of changing (both in your mental models, and the technology itself).
All this really means is you're right about the build a little/sell/build more cycle - but what you build is less important than what the customer says. The first build is often just a means to start an interesting conversation about the customer's true problem, and you might need less technology for that than you initially assume.
If you can convert random users to paying customers then you're on to something. Otherwise your project is most likely going to flounder. Some people may think it's dishonest but without verifying you're going to spend weeks to months working on something that will never be used.
You can definitely put up a product page where users can enter an email address and start building up a mailing list, but obviously mailing list recipients are not guaranteed to pay you money.
I do think that building a UI prototype that you can click through is a good first step for complex software.
We were the 'technical creatives' and with an idea and a cost, and went out to find people who we could sell it to to enable us to build it and make it better for all the users. As our product was video crossover - this was a little easier as people are used to paying for a video before you've made it, but video was actually a very small part of what we were offering - and sometimes not at all.
But selling things that have been bukt yet shouldn't be impossible, or even hard - as long as you can show a track record of delivery.
so, setup a landing page with a "send us our email so we can tell you when our tool is read!" form and see if it attracts people.