Sun and Moon didn't launch in all regions simultaneously so the 7.2m figure is an estimate taking first week from each region.
NA says "under two weeks" though so it's probably less than 7.2m
It's definitely less than 7.2m.
If we go with a dropoff of 75% from week 1 to week 2 for NA, we get roughly 3m first week and 0.7m second week. Remove that 0.7m from the global total of 7.2m and we get 6.5m.
There's another caveat though. In the UK thread about Pokémon sales I spotted the information that Sun/Moon had an unusual Wednesday launch in Europe instead of the usual Friday launch for the region, so in at least one region the first week constitutes two days more than Sword/Shield had.
In any case, we are dealing with a comparison that isn't 1:1, so we won't get around making estimates to get closer to a 1:1 comparison. I'd say Sun/Moon had a bigger opening than Sword/Shield, but ever so slightly.