It’s not necessarily the case that deferral = rejection. I know at least one kid who went to Harvard, and several who went to Yale, who were deferred SCEA and accepted RD. Of course, the vast, vast majority of deferred SCEA applicants are ultimately rejected or waitlisted, but that’s true of all RD applicants (other than athletic recruits and movie stars, or the equivalent). Applying to Harvard means that you are overwhelmingly likely to be rejected. The only think you could do to make your chances any worse would be not applying to Harvard.
However, deferring SCEA applicants doesn’t change how Harvard reports its admission rate at all. If Harvard and Stanford get the same number of total applicants and issue the same number of acceptances to fill their respective classes, they will report exactly the same admission rate, even though Stanford rejected 80% of its SCEA applicants without deferring them and Harvard only rejected 30% and deferred 55%, rejecting almost all of them later. That would mean that Harvard probably accepted a slightly lower percentage of RD applicants than Stanford, because more of the people it was accepting RD were deferred SCEA applicants. But Harvard generally doesn’t want to publicize that, and goes to some lengths to make it difficult to figure out.
Also, Harvard will accept a lot more than 600 people in its RD round. Generally, it accepts a bit more than 2,000 people to fill a class of 1,650. If it accepted 940 people EA, it will probably accept around 1,100 people RD. Of the people Harvard accepts, about 400 decide to go elsewhere.