Nanodiamonds rain from the sky - or not?

In reference of the article Diamond clues to beasts' demise I have to add that if you were to throw a giant ball of frozen methane into the planet Earth, you would not only get lots of diamonds as methane is known to produce diamonds on thermal decomposition under low pressure, but also you would get no traces of the hydrogen from methane, as water is water and it flows without leaving many traces. Also, burning methane would spread the cloud over the continents as the smoke and diamond particles too would have to be removed by precipitation of water - rain. It also explains the rapid climate change - something reminiscent of nuclear winter when fine smoke inhibits the energy input from the Sun.

You would really need only... small amount of frozen methane in the asteroid to impact Earth, as the plume of smoke would already start in stratospheric heights, making the sunshine blockage very effective from the start.

What if the centre of the blast was above what we now know as Gulf of Mexico? Would it cause double-disaster of lower temperature millenium with insane rainfalls at the beginning? Terrifying image, isn't it?