Indeed, that's very true. I've experienced first hand trying to market something that I haven't finished imagining. But that's the whole point, you have to start before you've finished, even though it is hard.

It's difficult to market in general, at any stage of doneness. Marketing can be hard, boring, frustrating, and not fun. It's especially tempting as a dev to think that if you just make the product better, people will see it, and recognize its amazingness, and it'll go viral and market itself. It's tempting to keep adding features and making the product better. But that strategy doesn't work, it's important to buckle down and do the marketing, otherwise nobody will ever know about your game or product, no matter how good, and no matter how done it is.