If I were the cynical sort I'd say it sounds like she was trying to catch you out.
Yep, that's exactly what I thought - I must be the cynical sort, then 
I think she deliberately targeted you (and has probably done it to many others) and set you up to fail on purpose. That's why the play date ended up being at your house.
Her twin hobbies are performance-parenting and performance-veganning. When parents ask if her DD has any dietary requirements, if she replies in good time, like any normal person (with her preferences) would, that the girl is a vegan who only eats whole foods, this gives you the chance to accommodate her requirements, all good and deprives her of any attention and ability to point-score.
I think her (and her husband's) game plan was/is to deliberately put their DD in the position where an unsuspecting parent will feed her ordinary food and then throw their hands up in horror. "I said she likes FOOD - how could you possibly consider anything with animal products in it or that's been processed as actual food?!" In the same way as most normal people, if asked if there are any games their children particularly don't like, wouldn't dream of saying 'playing in the middle of the motorway' as they'd take it as a given.
I agree that there may also be an element of control in that she sees play-dates as free childcare and knows that her DD will gladly go on them without a fuss, because she knows she'll get normal food she'll like that her parents don't let her have at home.
She knows exactly what she's doing - she did from the very first conversation about the play date. Definitely turn it back on her with the earlier PPs' suggestions ("When I asked about dietary requirements and you said 'None', you actually meant ' Vegan and Wholefoods only' - you need to work on communicating better, especially with such limited preferences.")
She'll still brazen it out and pretend that she would never have expected you to consider anything else as proper food, to which I'd just non-maliciously point out that it's you and 99.9% of other parents - and even most of the rest of the 0.1% would be sensible and non-scene-making enough to simply mention it beforehand.