The Roman Catholic church has something like a billion adherents. Scientology has well under 100,000, but let's be generous. That's 10,000 Roman Catholics per Scientologist. So five high-level defectors from Scientology is like 50,000 high-ranking Catholic clergy all defecting.
I got curious and googled. There are just under 34,000 Catholic bishops in the world, and that should include all the archbishops and higher ranks, except perhaps for a handful of cardinals. So for 50,000 ranking Catholics to quit, they'd have to dip well below even the local bishop level, to the point where there would be essentially no-one left in the hierarchy at all.
So maybe this is actually not such a silly point. Melton is way off, comparing the Scientology exodus to the departure of a mere five cardinals. It's more like having every bishop in the world reject the Pope, while tens of thousands of formerly prominent clergy, including the entire college of cardinals, have been locked up incommunicado for years in a Vatican dungeon, eating gruel and licking floors.