For me I do it this way:
1. Get a sprite system, audio, sfx, save state, game loop solution going for the platform (GBC). Usually something is available like Cocos.
2. Create the game! Sprites, animations, sounds, game code for all parts.
Then I test play, and iterate until it’s ready to share.
Starting from scratch like this website is like starting from step zero, not step one.
Yeah, nooooo. Cocos2d is a relatively heavyweight game engine aimed at platforms with bitmapped graphics and lots of CPU and memory. It's completely unsuitable for an embedded system like the original Game Boy, which uses hardware tile/sprite graphics and has very limited compute resources (4.19 MHz Z80-like CPU, 8 kB SRAM).
2. Create the game! Sprites, animations, sounds, game code for all parts.
Then I test play, and iterate until it’s ready to share.
Starting from scratch like this website is like starting from step zero, not step one.