Apples and oranges.
The Whitworth could be shot either with a fitted hard lead hexagonal bullet or a round soft lead bullet which would slug up to the hexagonal bore. Any road, the Whitworth was superceded by other rifling plans within five years. The .451" 20 twist layout was the big improvement over .577 Enfield, not the rifling.
Modern "polygonal" barrels are made with a cross sectional area close to that of the bullet so they are a force fit, just as with land and groove.
I figure "polygonal" barrels are easier to make by modern hammer forge machines than "crisp" land and groove.
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