There's slippage of claims here, maybe. It's clearly not the case that every record with wrong data is fraudulent, and I don't think Musk believes or said that (though maybe he did, the dude tweets all the time). We certainly agree that it isn't the case.
The claims being made as I understand it are that the US SSDB has a lot of obviously wrong data in it, that bad age data is frequently a sign of fraud as proven by the audits of other countries, the age data is genuinely bad and not a result of misunderstandings, and therefore that Musk is likely correct when he argues there is a lot of fraud in the system.
What does "a lot" mean? It's vague but clearly not 100%. At the same time, if the numbers do transfer then less than half a million people fraudulently claiming pensions is quite a lot! And that would be just one source of fraud, one of the easiest to detect without effort. There will be plenty of people under the age of 100 who have died and weren't registered either, or people who never existed and so on. So whether we're talking millions, or a million, or even with a very restricted criteria less than half a million. In ordinary English, it's a lot.