Reading the article again it struck me that the author was trying 440 grain bullets with black powder and being unsatisfied with attained velocities.
This does not surprise me because the traditional period loads were typically using 340-380 grain bullets (Westley-Richards even offered 300 grain loads) to achieve express velocities.
Now, Holland & Holland did offer .500 Express 440 grain loads with BP and regulated some rifles for such, but that was Hollands thing.

That the author found success using Nitro for Black loads (H4198) with 440 grain bullets is not unexpected because 440 grain bullets were the accepted NfB Cordite load that would regulate in the older .500 BP Express rifles.

As noted; his is an entertaining story even if sprinkled with inaccuracies and head scratchers.....not a great resource for the uninitiated.
- Mike