The idea of the hostess sending her a specific recipe is bizzare. And actually, that is not a pot luck. Pot luck specifically implies everyone bringing what they want (maybe within specific categories). In this case, the hostess planned a meal and then engaged her guests to cook it for her. Not cool.
I really gotta wonder what the side dish was that LW had never heard of. Was it the dish & recipe itself that was unfamiliar or was it the ingredients? Main ingredient or a specific spice or condiment? In other words, how big a deal was it for LW to just go ahead and make it, or make something similar?
I would definitely be put off by such an invitation, but once I'd accepted the invitation, I'd do my best to go with the flow. I would either go ahead and make the recipe or find something as similar as possible to make, letting the hostess know what I'd be bringing instead.
And I probably wouldn't accept another invitation from her without clarifying all the terms in advance.