It does fall in line with other estimates, however. Look at
https://realmpop.com/us.html, filter to level 120 characters only, and it's around 1.7 million. So either this API leak was, indeed, only for one region, and in that region nearly
every single person who hit level 120 stayed subbed, and never leveled an alt to 120, or.. It was for both NA and EU (Which has about 2 million max-level characters), and more than half of those max-level characters are either alts, or unsubscribed players.
And, as a personal anecdote, more than half my friends who were playing WoW during Legion.. In fact, more like three quarters, have unsubscribed themselves. Though given we're all RPers, that might be something of a slippery-slope effect, where one migrating to FFXIV draws another, which draws another, etc., regardless of WoW's quality.