"FINAL FANTASY XIV: A Realm Reborn is estimated to have 1,854,888 players per day this month." (Source:
https://mmo-population.com/r/ffxiv)
The OP's graph from Steam, rounded up, is 26,000. That's about .014%, which is certainly way below 10%.
Stats indicate an average of 3,460,103 active players in Januay 2022 (right after 6.0 launch), and 3,682,079 active players in Jun 2022 (after 6.15 release with new Ultimate). It appears that the Yearly population chart has a dip down to about 600,000 active players in the mid-March to early-April time frame, before the 6.1 release.
Yeah, that means an 80% drop in the active player population for the last of a 4 month lull in new content, followed by 600% increase in active player population when the new patch came out.
And yes, the numbers are as dubious as any other when talking about populations for online games, although I trust them more than I do some count of the Steam gamer population statistics that say nothing about all of the console and PC users of FFXIV who do not play through Steam.