It IS physically possible to revive cryonics patients--as in the laws of physics would permit it, even if we don't have anything near the technology required, yet. This isn't a bold claim but rather a statement of fact with respect to information preservation of the vitrification process, and long-term storage at cryogenic temperatures, both of which are well studied in the context of things like organ donor preservation.
So if you're claiming that cryonics has "a snowball's chance in hell" of working, then I presume that either you are (1) ignorant of the science, or (2) making some sort of statement about long-term storage prospects or the credibility of the organizations involved.
But neither you nor the grandparent actually made any specific claims.