I have no doubt the old software will work with older RAM, and if it reports problems, the problems are real. As I noted above, what was certain in my mind was that the RAM was bad.
But old software can still have bugs and newly discovered bugs and the fact it is no longer under development means any bugs are probably not being addressed. A program that is still in current development is likely to kept current.
I do, however, disagree with your assessment that Windows Diagnostics is not worth the time. While no software based RAM testing program is 100% conclusive (that takes specialized and expensive test equipment), Windows Memory Diagnostics uses the same testing protocols as other testers. It is basic in the way most integrated Windows tools are basic - that is, no fancy UI. It has been around since Vista. If it was not worth using, MS would have dumped it. Like other RAM testing software, if Windows Memory Diagnostics reports the RAM is bad, it is bad.
That said, it would not be my first choice. MemTest86 from Passmark is - which BTW, comes from the same roots as MemTest and MemTest86+.