Cooking rhubarb with soda
The whole quote is 


  Cooking the leaves with soda can make them more poisonous by producing soluble oxalates 


I can't tell you if the claim as a whole is true. But if it is true for the leaves, it is true for the stems too. 

Rhubarb contains oxalic acid and its salts which are created by the acid reacting with different metal ions such as calcium and magnesium. It's true that these salts have different solubility and bioavailability. And both the leaves and the stems contain oxalic acid and salts, just in different concentration. 

So, if cooking the leaves in soda turns the insoluble oxalates in them to soluble ones, then the same will happen when cooking the stems. For the metal ions, it's irrelevant if they are reacting with an oxalate ion which used to "live" in the leaf or in the stem. Anything which makes the leaves worse will also make the stems worse.