You are correct, but that limitation applies to memory testing in all environments (Windows, DOS, Linux, etc.). However, depending on the environment (DOS for example) the diagnostics can access a greater percentage of the memory at one time. This isn't only true for our diagnostics software, the only testing method that has access to a greater percentage of the memory is with the use of a stand alone memory tester which cost over $4000, and is only capable of testing a single memory stick at a time.
If you want to keep using the system without changing the memory, I would recommend running regular memory tests to make sure it doesn't begin to get any worse. If there is only one bad sector, then you will likely be fine for the time being.
Please let me know if you have any other questions or issues.
Thanks,
Kris