It may not seem logical but it is nevertheless true where low efficiency traps are concerned.
Would you stand open dishes of honey, fondant, syrup etc on the top of your hives? Would this be adding novel attractants to the apiary?
I still think you don't understand the wasp robbing hives scenario. In an ideal world, every hive would repel all wasp scouts... but they don't.
Remember that each hive holds many kilos of honey, with not all of it sealed. Hives are actively ventilated, honey smells good... HM's 'honey plume'. On a warm evening after a good day's forage, each hive might have several litres of nectar to ripen, with further sustained ventilation as the water is evaporated. You can walk into the apiary and smell the ripening... and that's with our ineffective human noses, not the highly tuned senses of a starving wasp
So each hive smells like an 'open dish of honey'. Think of the smell coming from an apiary of 12 or more hives. Adding a couple of traps with 200ml of sweet/ferment smell is a distraction not the main attractant. Hence my view that it's illogical to say that adding a trap makes the robbing worse.
But the issue here is what I would call 'wasp pressure'. Single scouts that are batted away by strong hives with correctly constructed entrances in the absence of traps and other superfluous sources of attraction represent little if any 'wasp pressure' on the hive. This is the ideal situation.
Adding extraneous attractants to the apiary including 'low efficiency' traps increases 'wasp pressure' because it draws wasps into the area that would otherwise not remain in the area having not been able to feed successfully in the area. If the 'wasp pressure' around the hives becomes too intense then the hives are at real risk of being overcome.
The moment a hive is infiltrated by wasps, it becomes an inefficient trap - reward & release. After prolonged robbing, a honey bee colony will cease guarding the entrance, as if demoralised. It becomes a completely inefficient trap - just like your "dish of honey, fondant or syrup". They will simply allow the robbers to come and go as they please; the colony is doomed if the robbing persists. Even an open rapid feeder (as posted earlier) is more efficient at trapping than an overwhelmed colony; the feeder clearly drowns some of the wasps visiting, whereas the overwhelmed colony lets them all go.
I found just such a colony last week - a couple of hundred workers around their queen and a tiny patch of brood, wasps coming an going unchallenged, stores completely stripped out.
Just one colony in this state starts a wasp feeding frenzy until stripped. Having then familiarised to the apiary, the wasps will then mob the surrounding colonies; if they find one they can get into, the process continues.
Wasps will strip nucs in a couple of days, moving from one to the next. Mating nucs are even worse, I've seen those stripped out one a day, even the strong ones with entrances reduced to a single beespace.
So, knowing something about the behaviour of wasps around apiaries, I stand by my point that it is illogical to say that adding a trap makes robbing worse. Any trapping alleviates robbing pressure.
The concern for beekeepers is that wasps are going to be attracted to hives, they will persist until they find one they can rob, and they will then strip that colony and any subsequent ones they gain entry to. Yes, there are hive entrance designs that reduce the risk of a colony being overwhelmed, but selective, seasonal trapping of wasps at this time of year is an effective defence for colonies. The trick is to find a bait that attracts wasps not bees, and a style of trap that fills up - the faster it fills, the more efficient it is, and the more effective it is at removing robbing wasps from the apiary.
