Just from what I know about Fredersdorf and Lehndorff, I'm also totally voting for projection! <3 Could also have been projection from someone else -- like, I could totally imagine some random noble who was jealous of Glasow saying to Lehndorff that it might have been a thing, and Lehndorff going "oh, yeah, makes perfect sense!" and writing it down.

But, yeah, sick and tired and about to get a nurse, really Fritz married, I don't think jealousy would even have occurred to him.