Strong marketing plan is true, but not accurate. I think a lot of people's marketing plan is to show it on HN/Product Hunt/Indie Hackers/Reddit entrepreneur groups, which is a horrible marketing plan.
I think the marketing plan is part of the project design - you should have it before you build it. Are you targeting remote workers? That should be part of the UI, the features, etc. Do you plan on marketing via social media? Your app should have lots of share buttons and the formatting of the content should be shareable.
Every time someone brings up the discussion of whether an app is web, iOS, or Android, my answer is always how that links to marketing. Some people are better at some forms of marketing, and that has to be in synergy with the product.
However, if you need to throw money at it, it's also likely not doing a great job. Sites like Google and Facebook launched with no paid marketing, even though they spend a good amount of money marketing now. It's not that they didn't have money, but not using paid advertising is a good way to gauge how strong your product is and give you time to patch it.