Seems part right, and part wrong, to me.

I think they're off in their question that asks "Would a comet impact create enough pressure to create diamonds?"

I happen to believe that diamonds are rather common in the universe and a by-product of stars going super-nova. If true, comets can inherently be, in whole or in part, made of diamonds or comprise diamonds.

So their theory stands rather well, imho. Even though a question they ask seems questionable, imho.