Having a good game is table stakes for sure but I'd not ignore what the OP is saying about marketing spend relating to marketing effectiveness. You can't buy a frontpage slot on Steam but you can do simple things like adding translations that will get your game shown in more regions. Which is quintessentially a marketing activity.

Then you have the ability to affect things through off-platform marketing. If you make use of that to find success the organic discovery on Steam compounds the result.

A good game is table stakes though and good relates significantly to the market segment your game is in. Understanding that is also part of marketing the game.