The problem is that their primary claim isn't centered around injury, specifically alleging a transgression under a statute. It is specifically because they are alleging that Act 77 is unconstitutional under the PA constitution. That is a case that can inherently be brought at anytime because the injury is its very existence not in specific harm. Thus by not doing so, despite the fact it past more than 180 days previously and was in fact the rule under which the primaries were conducted, it terminally weakens their case.
If a law is unconstitutional, it is unconstitutional the day its passed. If they were alleging fraud or any other specific harm as their primary theory, than that lawyer and you might be correct.