Tofu is a protein too! The great thing is that all these things are easily interchangeable, so you use whatever you like.
Your post did remind me of an advanced technique. If you are cooking chunks of protein, the best method is to first fry the protein, then take it out, and then follow the original recipe. Why? Because you want to get that browning, and you only get that with direct contact between the protein and the pan. (This is known as the Maillard reaction.) But if you leave the protein in the whole time, it dries up faster than the vegetables (they have cell walls that are hard to soften).
Your post did remind me of an advanced technique. If you are cooking chunks of protein, the best method is to first fry the protein, then take it out, and then follow the original recipe. Why? Because you want to get that browning, and you only get that with direct contact between the protein and the pan. (This is known as the Maillard reaction.) But if you leave the protein in the whole time, it dries up faster than the vegetables (they have cell walls that are hard to soften).