In the patch group editor, if I check only software distribution under patch type and nothing under vendors and families, I see lots of software distribution patches. But if I check Google or most other options under vendors and families, I see no patches. If I check only Microsoft, I see many (all?) non-Microsoft patches. Software distribution patches are also missing if I check other options under vendors and families. Using the search box to do filtering is a workaround.
Most Software Distribution type patches are associated with the OS so filtering needs to be based on OS and not the product. Searching for product name with Software Distribution enabled with no other filtering is your best bet.