The intent behind the "things are always considered derivative of Yal Stuff" clause is that you're protected, so people can't buy the same asset pack, quickly throw together a game, and then claim you infringe on their work. (Credit is optional unless stated otherwise - and so far no asset pack I've released has had a different credit statement - so you don't need to credit me for neither directly derivative stuff nor stuff derived from your own work)
The second point you bring up overlaps a bit with the first one but also is there to protect me from liability in case someone makes, say, a Fallout fangame and gets bonked by Beteshda - if someone uses intellectual property they weren't allowed to use, they also broke the EULA and thus I can't be held responsible for it (and conversely just because two people bought the same engine/asset pack doesn't mean they're allowed to steal each other's intellectual property).
"Resource Licensed Stuff" is asset packs / source code and "Executable Licensed Stuff" is games / software tools, so yes, JamTactics Engine is a Resource Licensed Stuff. (Maintaining one EULA is complicated enough so I use the same one for both games and asset packs instead of having them separate)
Sorry for making this thing so complicated, but generally it's safer to explicitly state everything in a legal document even if it's supposed to be obvious / common sense.