Not on desktop Safari, either. Can see and move the brush but can't actually paint. I assume it's something with the drawing process itself, as I can't imagine someone managed to write a click handler that's not cross-browser, so surely that's firing correctly.
I know Apple’s support of various WebGL features has generally lagged behind Chrome. Though even when there is feature parity shaders seem to run slower in Safari than in Chrome (e.g. a shadertoy shader that north works in Safari and chrome will run at half the speed or less of the chrome one). It makes me think that Apple is doing some GPU throttling for web use cases, maybe for battery or fan speed/heat reasons.