Note - I'm not going to specifically defend what you're poking at, but I think that your defense here is based on false premises.
They don't need to be singled out. The goal would be to reduce the vote totals for Democrats by 120,000 more than reductions in Republican votes, to use the numbers there arbitrarily. Such could be done by going after just about anything and anywhere that is more likely to be used by Democrats. Mail in voting, for this year specifically, for example, seems to be much more skewed towards being used by Democrats this year in most states, hence the Republican campaigning and legal attacks against it. That PA naked ballots thing was just one front in that political offensive, not even remotely based in principle.
Personally, though, I'm a bit more concerned about the scenario of the PA GOP trying to sabotage mail in voting counting, then using all the ballots that aren't counted because of their sabotage as an excuse to just appoint their choice of people to the Electoral College - as the Trump campaign has been in discussions with the PA GOP about.