in the scenario that replicated the endowment effect successfully, the buyers never got to hold mugs, while in the one that did not, they got to hold mugs and were then asked if they want to pay to keep them.

That was not my interpretation of "Buyers were allowed to inspect the mug of the seller sitting next to them." I'm pretty sure (70%) that this means that buyers were allowed to physically hold the mugs. This, at least, was exactly how we did it in an in-class demonstration of the endowment effect (and indeed the WTP-WTA gap was present).