Many of us are old enough to remember waxed and/or oiled cotton tent fabrics. There are a few issues you might want to think about, for example:

How fast would you like your tent to burn up if exposed to flame? Seriously.

Are you prepared for the tent leaking wherever you touch the inside surface while the outside is wet? As your source says, pores in the cotton remain open but water will bead up and run past the pores, leaving it effectively waterproof. What it doesn't say is that as soon as you touch the inside (or your sleeping bag does, or your baggage, or your hair while you sit up to get dressed) water will be drawn through those pores by capillary action, and that spot will continue to leak until the tent dries out. Coated nylon doesn't do this.

Last, although wax is not poisonous, linseed oil, turpentine, and other solvents are. Some materials, like linseed oil, support mildew. You might not want to go to sleep enveloped in fumes or spores.

None of which means don't do it. But do a flame test first, and think about what it would be like to be inside a burning tent.

Hope that's helpful.

Mark

Edit to add: I don't mean to be insulting, so take this for what it's worth: wax and solvent on a stovetop is a famously effective way to start a house fire. That's why your source says "double boiler." Use the double boiler, but if you're really cooking up wax and terps, do it outside in a controlled environment, still using the double boiler.

Or not: your choice.

Edit once more to add: I seem to be having some trouble leaving this one alone. If I were you, I'd merely cover the whole tent with a blue, orange or yellow woven plastic tarp. Presto: it's waterproof.