Board Thread:General Discussion/@comment-27565137-20160130014722/@comment-26091912-20160202093406

^ That may well be evidence for your theory, but it's out of context. Urahara is trying to convince Chad and Orihime about the existence of Soul Reapers and Hollows; something which is central to the series. Chad and Orihime are having enough trouble as it is believing the man. Revealing to them the existence of an all-powerful orb which can fulfill the wishes of those around it (which even he didn't know about yet) would completely alienate them.

Urahara once believed the Hogyoku's power was to control/break the boundary between Hollow and Shinigami. But as Aizen pointed out, as its creator, that's what Urahara chose to believe when he stabilized the souls of the Visored. It would make sense that Aizen knew more about the Hogyoku than Urahara, because he created one first and obsessed with 'completing' it for over a century, while Urahara sealed his away because he was afraid of its power.