Valid points, and I certainly see and respect your logic. I do think the approach you choose varies greatly with age. If you try this, messing with the dog's food, with an older dog who is pretty serious about gaurding his food, you're likely to lose some blood. With a puppy, not so much. So there is that!
I can actually understand how "messing with a dog's food" in the way you're describing could teach them not to guard too...provided you start with a puppy AND give them no other reason not to trust you. Again, if it works for you and your dog it is the right approach!
That being said, I'm very much in agreement with @Sabis mom
in that, I've yet to see a dog that can't get over resource guarding food from any human (dogs don't count because sometimes they can't be trusted for good reason LOL!), with the one exception I mentioned previously (the complete nerve bag that required medication to just get through the day without self mutilation and panic).
I certainly wouldn't try - or suggest to anyone else - to just "mess" with an adult resource guarding dog's food - without desensitizing them first and building up some trust. It would be a process and it would involve lots of hand feeding most likely.
This, like many other behavioral issues, can be extremely difficult to overcome if/when you attempt to "confront" the issue directly. More often than not, again IMHO, the problem is often best approached indirectly. A resource guarding dog, for example, often will have poor impulse control across the board. So working on impulse control in non-threatening ways can help. Likewise, general obedience usually helps broadly with other unwanted behaviors. And working on these things with the dog helps build trust, so it's a win win win.
To me it has always been fascinating how several vastly different training approaches can achieve the same or similar results. Trying to understand why or how that can work, I feel has helped me to better understand the canine mind...inasmuchas we can ever hope to do so