Some say that it actually means that DC isn't planning to bring them back, but...pay attention to the epitaph:
Why would they have "grandparents" on their epitaph if they died well before Jon was even born? Honestly, would Jon's birth prompt Clark (or anyone) to go back and add "grandparents" to their gravestone? I don't know many people who would do that.
I may be reaching, but do you think maybe this was a little hint by DC that they're bringing them back soon (or at least bringing back Martha since she was the only one alive Pre-Flashpoint anyway)?