It depends on the idea, lead quality and market. Organic search to conversion of 3-4% is awesome, super targeted leads maybe 10%. Of course numbers generally are higher for things like newsletter sign ups etc vs handing over cash.
I'll admit I haven't read your link but if you were doing business validation and 1 in 5 people were saying yes and the market is large enough then those are good numbers. If your market is super small in the world then I'd say no. Of course there are a few exceptions to that rule.
If I remember he had a free ebook that talked about the manual process and the effort it takes to do it manually.
1. get 10-20 people setup on a $5 beta for a couple of month in exchange for a testimonial if they like it.
2. for the people that it's not working for see what the problem is. fix it.
3. work with them on building your ebook. offer to cross promote(give them a link) for some of their design advice, tips for finding work, closing work in the ebook to get some content.
4. try build some good relationships with the initial users.
Clickable, https://www.indiehackers.com/@patrics/i-was-excited-about-an...
The hard task for designers is finding good clients and good projects. Talking to designers might provide some insight into that problem, but it will never solve it because it is talking to the wrong side of the marketplace. It's akin to beginning a gold mine business by talking to jewelers.
There's almost certainly a market if a person can find a gold mine of design clients because there are existing markets for clay mines of design clients. The problem is that one of the things that makes a golden client from a designer's perspective is a relationship that spins off repeat business...and by it's nature that means a client gold mine has to continually find new high quality design clients. That is going mean very hard work identifying and talking and selling on a case by case basis (and not building a platform or really building much of anything at first).
It is also going to mean finding Tiffany quality designers. And that will require identifying and talking and selling on a case by case basis, too.
Good luck.