You are right.

Quite a few years ago I was at the Antique Arms Show in Las Vegas. Holland & Holland had a prominent setup, with one of their .700 Nitro Express doubles as the centerpiece. They were gracious to let me handle it. I asked the attendant why they didn't just make the rifles as a 12 bore Nitro Express. He looked at me as though a light had just come on and replied, "I don't know."

Could be they didn't want some idiot slipping one of the new NE rounds into an antique 12 bore rifle and letting go. But interchangeability of NE and BP rounds didn't bother them 115 years ago. They did exercise a little caution, though. Here's the rib inscription under the forend of my Purdey .500 BPE (made in 1894 after the advent of smokless):

And look at the red stamps on the case label and charge card:

Curl