When you pressed the claim the information in the tooltip says, just as someone pointed out, that he will only become your vassal if he's of your dynasty or if he's a de jure vassal of yours. In this case he was neither so the end result is the correct one. The error here is the last line in the pop up which is clearly wrong - and has now been fixed. What you could do if you had wanted him as your vassal is to give him land within the de jure part of the HRE and then press the claim. In tat case he would have ended up as your vassal.
Good to know. The main thing is that the information be presented to the player clearly.
For what it's worth, I'd include in the Tooltip "WILL NOT BECOME YOUR VASSAL"
or even
"WILL NOT BECOME YOUR VASSAL UNLESS YOU GRANT HIM LAND BEFORE CLICKING THIS BUTTON, SO DO THAT RIGHT NOW AND COME BACK AND THEN CLICK HERE".
The Inconclusive Peace events are frustratingly unclear about what just happened. As King of Croatia I was DoWed by Hungary on behalf of a (very recently created!) pretender to a Count title. I was very nearly in a position to demand that he relinquish the claim and take the full prestige hit, when I got Inconclusived for no apparent reason. Maybe the Count died, maybe the Byz Emperor cut a deal without me? If a player has that happen in a couple important wars it's going to drive some people crazy.
Have the rules play out however they're supposed to play out, but the more arcane they get, the more you're gonna have to give it to us in Captain Dummy talk.