It's extremely unlikely that the Mole Antonelliana and Philadelphia City Hall share the title of world's tallest masonry building. That would be absolutely ridiculous. I don't think that you could make them the exact same height even if you were trying. The only way that even seems possible to me is if they're both up against some engineering limit of habitable masonry construction, but honestly, that's stupid. One of them has to be taller than the other. So, I'm doing a little research into where these figures of 548 feet are coming from.
From what I can tell, the Mole Antonelliana figures are really inconsistent. In an 1897 engineering journal and separately a 1906 tourism book , the height is listed at 538 and 536 feet respectively:
Now, I read online that in the 1950's the original spire was destroyed, and if that's true it could have been rebuilt higher. But the contemporary numbers are all over the map. The website for the National Museum of Cinema (which occupies the Mole Anotelliana) lists the height at 167.5m (549.5 feet). A 2006 travel article from San Diego Magazine lists the height at 556 feet above the Po, which might not be the same measurement, but if it is, it's almost certainly wrong, because that would make it taller than the Washington Monument. And yet another article in an French travel guide from 2009 lists the height at 163m (535 feet).
It's all dancing around the same height, so maybe they are the same by some weird fluke. But in the interest of settling this tie, I've written to the Museum of Cinema, asking how they know their figure for height. If they're confident, then the Mole should probably be the tallest, but if their figure isn't sourced well, then it's likely that Philadelphia City Hall is taller.