Some places have more than one kind of sauce. They have one that's like this, and they have a more tangy version. It's similar to Westen NC sauce. It has less ketchup, so you probably would have had about the same amount of ketchup as you did liquid and a little hint of mustard. On my ribs I preferred them to be cooked Memphis Dry style then eat them with a vinegar sauce like this one
I used to run a Memphis BBQ restaurant (previously was the KM and pitmaster). Memphis BBQ sauce is NOT made with ketchup (never has been). A "BBQ expert" should know that. It's usually made with tomato sauce these days (with a little tomato paste added), but I've seen variations with crushed tomatoes and originally it was made with cooked, blended whole tomatoes (that's the style Henry Perry took up to Kansas from Memphis when he introduced BBQ up there). It's also made with molasses (usually blackstrap) and has paprika (usually smoked) because of the Greek influences in early Memphis BBQ. If you use ketchup, then you've made something closer to a Texas-style sauce, which is perfectly fine if that's what you like, but you can't call it Memphis-style because it's not.
