My time working on certification items in cabin configurations leads me to agree - it is legal as the basis is by qty of seats, but is practically limited by the No. of O2 masks available (flaps, don't forget that O2 boxes (should!) carry more masks than qty of seats - the issue comes if you have more than 1 infant in any 1 seat row). This would be written into airline SOPs somewhere. However, strictly legally it is OK.