Just becaause they are not listed in the SSDI doesn't mean they didn't fill out a Social Security Card application and receive Social Security benefits. The SSDI started about 1962 and mainly has deaths going forward from that year. It's also not 100% complete.
My grandfather died in 1963 and he's listed but my grandmother who died in 1971 is not listed. I received the SSA applications for both of them.
If you can provide SSA with their SSNs, they should easily be able to find their applications and provide them to you.