Qt and GTK I could see being seen as 'cartoonish' because the default themes on many distributions are fat, shiny, bubbly, and look like they firmly belong in the 2000s.
Cocoa, maybe because of the vivid blue? Or that person hasn't seen Cocoa since before they moved away from the glowing blue bubbles, although I think they still look nicer than Material.
It seems to borrow the icons and some widgets, but not so much the container elements.
Apart from that, I simply dislike the look. It oft reminds me of Windows' Metro due to the sharp edges and corners. I'd prefer Apple's UI design over Metro although I am a Windows / Linux user.
One more thing, looking at Android apps, they sometimes use many different color tones without any underlying meaning. Or such an obscure meaning, that I still struggle understanding it.
As for the vitriol, here's a quote from the comments on the above post:
"As much I agree with you about what it has given to the community, as I disagree about not being fair to criticise it. Every creation should be criticised, especially if it’s unquestionably used by millions. Otherwise we won’t get anywhere. Criticism is not bad, even if it is done harshly. Especially in the age of corporations and their visually subliminal effects on an entire civilisation, I cannot possibly promote discouragement to criticism like you do."