I think they were talking about how Crawdads like higher PH levels in their water. The only problem with not doing the test is that your could be introducing nasty chemicals into the water that could harm or potentially kill the Crayfish. Yes I will agree that a higher PH is good for them to molt but chasing PH levels shouldn't be a priority, keeping the PH levels stable is far more important then chasing a certain PH level. As for the vinegar test it's just to test if the rock is water soluble or not, if the rock fizzes that just means that the rock will dissolve in water releasing anything and everything that is contained in the rock over time. That means any poisonous chemicals that "may be" trapped inside the rock will slowly be released into the water.
If your looking to keep a higher PH level without any risk to the Crawdad, I would recommend using crushed coral substrate. It'll help raise the PH levels without adding any unknown elements to the tank. Also if you add wood to the tank expect the water to turn a reddish brown color, temporarily, as it's just releases tanins in the tank, tanins are beneficial in certain situations but will also lower PH temporarily. So in other words, crushed coral raises PH permanently while wood lowers PH temporarily.