I can't be bothered reading the link, so if this is repetition, sorry...
SOHC - single heavy metal camshaft with big cam lobes on it, two big heavy valves per cylinder.
DOHC - two smaller metal camshafts with smaller cam lobes on them, four smaller lighter valves per cylinder.
Result: DOHC
(1) uses less engine power to make the lighter cams spin up;
(2) uses less engine power to push the lighter valves down;
(3) requires smaller, lighter, springs to push the lighter valves back up.
DOHC revs up quicker, all rotating and reciprocating parts weigh less and can therefore move faster so engine has a higher red-line, therefore generates more power.
Any time you shave weight off of a moving piece of your engine, you can raise the redline and, therefore, increase the horsepower.