But that encounter works just fine in pf2e, the trap has an exp value, the guards have an exp value.
If that combined exp value is within the encounter guidelines, there's no special reason it should be a TPK. If they were meant to each be hard on their own, of course combining them would make them too hard-- but you can adjust that easily enough, the game gives you the tools to do so.
If you could combine them in those other systems without thinking about it, than those other systems probably "collapsed" in the sense that the encounters would have been jokes individually. Like, offered no resistance at all, which is a pretty accurate summary of 5e, from the years I spent GMing it.
Conversely, if its obvious the guards wpuld attack, them congrats you just set the parameters for a single encounter, budget accordingly.
Look, if you and Zapp want to hate on the system (regardless of disclaimers to the contrary) thats great and all, but you're gonna get called out, "overdesigned, bloated mess" indeed.