I see your conceptual similarities between IRS and repos. But I wouldn't call it a synthetic repo. This is because it can only be an uncollateralised loan, as the security is issued by the borrower, not a third party as in a repo. So no collateral. By contrast, one could make the synthetic repo argument if the borrower borrows a security (or gets otherwise hold of it) and then hands it over to the cash lender. But then its already really close to a real repo.