Alright let's start of with your title. There are currently 17 keystones availible.
Precision:
Press the Attack, Lethal Tempo, Fleet Footwork and Conqueror
Domination:
Electrocute, Predator, Dark Harvest, Hail of Blades
Sorcery:
Summon Aery, Arcane Comet, Phase Rush
Resolve:
Grasp of the undying, Aftershock, Guardian
Inspiration:
Glacial Augment, Unsealed Spellbook, Kleptomancy
First of all let's remove all of the keystones that doesn't see frequent use/are not meta.
Lethal tempo. Only ever played on adc's like Kog'Maw, Kalista and such. Those champions has such a low winrate/are not meta so the keystone doesn't qualify
Dark Harvest. Very niche do not really see it being being picked over Electrocute.
Phase rush. Underperforming. Picked as far as i know only on Ryze.
Glacial Augment. Underperforming.
Unsealed Spellbook. Rarely gets played.
That leaves us with 12 total keystones. Speaking purely statistically, shows that there is a 0,83333... that each remaining keystone IS included in a normal game on SR
So seeing atleast one player utilizing Conqueror should not come as such a big surprise.
"The fact it shows up on champions that don't even have access to True Damage normally"
One of the goals they had with Conqueror was to make sure that it wasn't good on champions that ALREADY have true dmg. Seeing as it is not picked on fiora and camille I'd say they did a pretty good job with that.
Not to mention but as of right now the top 10 champions for top lane only includes 2 champions that use Conqueror. (http://na.op.gg/champion/statistics, 2018-07-08)