But is standing out _just_ marketing? Or is standing out something that requires originality in idea (as well as marketing to allow buyers to see the originality)?

Marketing isn't just ads, it means customer research and creation of offers users actually want. So marketing a game starts with researching niches and creating a game people actually want to buy. And then of course also target them with ads.

Promoting a game isn't enough on it's own. To succeed today a game needs to have quality, market fit, promotion, luck, and probably a number of other factors as well.