What did you do to make it retina?

You need to manually create a canvas at screen resolution and scale it to fit the screen. A default canvas has pixels that are logical pixels, not actual pixels.

What does "retina support" mean? What is the benefit?

I can't tell, since I don't have a Mac, but looking at the code, it implements a note from the Safari documentation for canvas [0]. I assume that if you don't do this, it either is very small or very pixelated.

[0] https://developer.apple.com/library/archive/documentation/Au...

It certainly looks much less blurry on my Pixel 7 Pro with this version.