I mean a project is something you do in your leisure time (like on a weekend) and it's meant more to show off your coding skills or scratching your own itch or maybe like gaining the pleasure a painter gains from drawing a picture.
A product otoh is a very different beast. It does need a project but that is like may 10-15% of it. The rest is a whole new world of selling, maintaining, growing which has things like content marketing, link building, SEO, optimizing Salescopy, sending out emails, building an affiliate program, recruiting affiliates, A/B testing, conversion optimization, reducing attrition, giving bonuses, doing discounts and coupons, cross-selling, upselling, funnels, analytics and a hundred different things.
The product makes you money, the project gives you pleasure of coding. For new founder and especially programmers trying to make a buck selling online services it can be a easy pitfall.