I was thinking more that Ryuken and Soken did believe that they were the last pure quincy family left considering the Kurosaki family was the only other pure quincy family left and that their general knowledge of any other surviving quincy was bad at best.. Though honestly all of the quincy lore is pretty conflicting from what's been revealed; almost all of them were said to have died before when the shinigami wiped them out, yet back when Ryuken was a kid, the number of pure family heirs was down to him and Masaki, yet there are still several "half breed" quincies out there apparently for Yhwach to suck em dry. Then you add in his whole army, many of whom I don't even think are nominally quincy to begin with and more just blessed with a portion of Yhwach's power (see the creepy one for example). But still, are the sternritter/vandenreich quincy or not? It's just kinda confusing, but then again this is pretty much a nonsensical arc just meant to extend the life of Bleach, so guess we'll roll with it as there was apparently nothing better to do than resurrect an old shinigami foe and power creep him to beyond Aizen at this point.