Thanks, shrinkrap. You answered it. I grew up on Del Monte, ergo “catsup”.
re dressings that keep, I keep Momofuko ranch and green goddess, as well as the above catsup based dressing in the fridge for well over a week. Just shake or wisk and serve.
A year later and I’m responding to this thread! 
I can say I’ve been eating this salad dressing for at least 25 years and haven’t gotten botulism yet! I’m no scientist but your rational makes sense to me. It doesn’t last too long around here. I go through a quart of dressing in about a month usually.
