Agreed. Extra analysis follows:
Slow and Purposeful says:
They are also allowed to charge in the same turn they fire Heavy, Ordnance, Rapid Fire or Salvo weapons.
There's two ways to read this. One is, "They are allowed to charge no matter what, as long as they fired a Heavy, Ordnance, Rapid Fire, or Salvo weapon." The second is, "They ignore the restrictions on charging after firing Heavy, Ordnance, Rapid Fire, or Salvo weapons." Here's my explanation of why the second is the correct reading.
Those are both equally valid readings, from a grammatical standpoint. The problem with the first reading is that it leads to the surprising conclusion that a Slow and Purposeful model has fewer restrictions on charging if it fired certain weapons than if it didn't. On this reading, a Slow and Purposeful model can charge after Deep Striking, but
only if it fires a Heavy, Ordnance, Rapid Fire, or Salvo weapon - if it fires an Assault weapon, a Pistol weapon, or doesn't fire at all, it can't charge after Deep Striking.
I think most people would agree that, if another equally grammatically valid reading is available (and it is), we should disfavor a reading that leads to such a counter-intuitive conclusion, especially when the second reading doesn't have any such counter-intuitive implications.