I’ll defer to Nate on the details but just wanted to add a couple of general thoughts.
My quick answer is that you can’t learn much from primary elections. They can be important in their effects–both directly on the composition of Congress and indirectly in how they can affect behavior of congressmembers who might be scared of being challenged in future primaries–but I don’t see them as very informative indicators of the general election vote. Primaries are inherently unpredictable and are generally decided by completely different factors, and from completely different electorates, than those that decide general elections.
The PA special election is a bit different since it’s a Dem vs. a Rep, but it’s also an n of 1, and it’s an election now rather than in November. Nate makes a convincing case that it’s evidence in favor of the Democrats, even if not by much.