Yup, I know about that and tried it some years ago. I didn't like the results. While I will concede that I may have done something wrong (like the proportions), nothing beats the convenience and simplicity of opening the bottle, pouring contents sparingly on a clean dry cloth, wiping on the body and be guaranteed of great results.
Right now, I don't see myself doing finishing work again in the near future, so the urgency isn't there to prompt me to go DIY with the oil finish mixing. But it sure wouldn't hurt to have a new bottle of tung oil on hand just in case.