No. To claim them as dependents, their gross (total) income must be less than $4,400 each and they have a much higher gross income due to social security. You can go through the IRS Worksheet for Determining Support. Line 1 shows taxable and nontaxable income as the starting point.
The meaning behind filing a joint return is that the other person is also low income such that there is no real tax liability or requirement to file. The IRS does not want you claiming a dependent married to a billionaire.
The stimulus is not officially a refund. It could be claimed as part of your refund if you did not receive it. If your parents are not required to file a return then they would only file a return for any money wrongfully withheld during the year or not paid, such as stimulus. They can file a joint return without issue.
You don't need to do a 1040x for them to file jointly. You will need to amend if you wrongfully claimed them as dependents on your return.
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