Yes, a 3 speed IGH is more efficient than most IGH with more gears.

But not than a derailleur only solution.

The problem remains that Brompton, for any reason; doesn't want to make a 135mm OLD rear triangle needed to be able to use a standard cassette (even if Kinetics has demonstrated that it has no impact on folding/folding width).

The 6s 2x3 was very well engineered with hub gear spacing and cogs sizes chosen to avoid duplicated gears and provide a more or less equal (but too big) spacing between gears which isn't the case with the 4x3 12s solution.
For me the 4x3 12s is a quick and dirty solution introduced to solve the too small gear inch range of the 4s P-line (and T-line, a 11s derailleur T-line would be a great bike, very lightweight and versatile, a unique selling proposition, there is no other folding bike at that weight with such versatility).