They're probably just afraid that you will be unhappy with the completed job, especially after it runs a while. I'd get the real boot and dismantle the shaft. You can't clean it without taking it apart anyway. And you can't properly lubricate it while assembled either. If you disbelieve me, try to clean one while assembled. Then take it apart and see all the gunk that was missed, and all the solvent left within. I've done this with joints that were off the car, using a parts washer and compressed air. I can't imagine getting it clean without even removing it, much less dismantling it.

I would note though, that when you dismantle the joint, you should put it together in the same way that you found it. Don't reverse the inner hub or the locator ring, and try to put the balls back where they were.