Measuring by last year’s sales doesn’t tell you anything about the numbers playing. I have figures bought 30 years ago that get played with regularly and I haven’t bought anything for that period since.
I’d suspect the ‘survey’ from ICV covers only those using proprietary games and similar systems, wargames rules where you have the choice of many manufactures almost certainly don’t even get a look in. I know we have arguments about what is and isn’t REALLY wargaming but this type of organisation is clear about what it includes and it won’t be independent wargamers using a variety of printed or home grown rules with figures collected and painted from a wide range of manufacturers to play historical, fantasy & sci-fi games. I’ll bet they don’t even consider most of the figure suppliers to be part of the same industry.
The answer is ‘we don’t know’ and probably will never be able to be certain. Probably GW type fantasy/Sci-Fi is the most played but does that count as a single ‘period’ or not ?
Also, for most of us, I suspect the answer is ‘who gives a damn’.