Heavy also means full of water in Arabic. And that is what he is not saying in the video. In the context of that verse that talks about thunder and lightning, the reference to heavy meaning is full of water.
This is confirmed by Ibn Kathir tafsir:
If you want to know what the Quran says about clouds, would you be shocked to know that the Quran says the sky could fall on us a ‘heaps of clouds’ – isn’t this just stupid?
The Quran describes the sky/heaven as a ceiling held up by Allah that can fall to earth or fragments break off and fall on someone. The earth’s atmosphere is simply made of gas and outer space is not a solid object.
Did you not see that Allah has subjugated to you everything on the earth, and the ships that sail at sea with His command? And He holds the sky so as it cannot fall on the earth except with His permission. Surely Allah is Very-Kind to the people, Very-Merciful. Quran 22:65
And if they were to see a fragment of the heaven falling, they would say: A heap of clouds. Quran 52:44
See they not what is before them and behind them, of the sky and the earth? If We wished, We could cause the earth to swallow them up, or cause a PIECE of the sky to fall upon them. Verily in this is a Sign for every devotee that turns to Allah (in repentance).Quran 34:9
Tell me where do you see Allah holding the sky? And what about other atmospheres on other planets? How can the sky fall as heaps of clouds? The Quran is full of 7th century desert dweller knowledge and thus is full of mistakes not miracles.