Copying an existing game is not developing it. In this example, you've "developed" nothing.

It's not hard to change a few features and make it different. What kind of dev are you?

Very true. Still doesn't prove that he gave zero thought to marketing early in development.

Thinking about something does not make it essential to the products success. Notch made the game good, which made the game popular.

This is false. Every industry veteran, titan, and novice knows this. You admitted you don't know much about the game development process, yet you make a claim without the evidence to back it up. Flappy Bird, a statistical anomaly which has not happened again, is the only such evidence.

Then give me sources of developers stating marketing is what makes a game good.

Games are fun because they are fun. Marketing comes after establishing that. You keep telling me that thinking about marketing once means it's essential, but you're wrong. So many products have been made with little to no marketing, that made it huge. If the product is good, it will sell. Make a good game, then market it. Otherwise you end up like EA where everything is a microtransaction, sequel of a sequel of a sequel of a sequel, copy of another games content. Battlefront and Hardline, for example. Goat sim was an original idea, I remember seeing it on reddit in '13 and the dev never thought the game would be anything. And now it's a multiplatform release.

Anyways, I'll leave you with one peace of advice. All the marketing in the world cannot save a bad product. Focus on making a good game, then think about marketing it. Otherwise you'll do a mediocre job at both.