If you know a bit of Greek, you will know the meaning of the two Greek roots "poly" and "morph"; it's not that difficult. I bet that many native English speakers do not know the meaning of that word themselves at all.

That's true. Polytechnic, polymer and polygon may well be known by educated native speakers. Morph is less commonly used in English words. Morphine comes to mind.

However, native speakers are far less likely to misquote idioms, such as 'It stinks to high heaven' as 'It stinks to heaven'. Non-native speakers make those kinds of mistakes, except perhaps for Indians!