Either Peanut or I would have told the server that this is not what we ordered, and with our dietary restrictions, this would make us quite sick before we got out of the restaurant. We'd do it nicely, but firmly, and request that it be exchanged for the original dish.
We've done this before, never saying a word about WLS, and have never had a problem (except this one place with a rude server, but we complained to her manager and we'll never be back there anyway!)
So many people have so many different food complications these days (was this made in a building that had at one time seen a peanut butter cookie?? Etc. ...) that it is completely accepted to just state "food reactions" and leave it at that.
Or you could get it packed and take it home to the devouring hordes waiting for something,
anything to land on their table!!
You handled it well for an unexpected complication. It gets easier the more experience you have with it. Eating out can still be fun, but it's more about getting out any more than it is about eating.
CT