For the most part yes it is true. You can do both at the same time, but you will not be overall pleased with the results - neither one will be done well.
For adding muscle, you need a surplus of calories. To lose bodyfat, you need a deficit of calories. See how it is hard to do both?
You need to pick one and go with it. If you are eating clean, and high in protein, you will likely add some muscle - especially if you are deconditioned. Additionally, you will likely lose bodyfat too. But this will end quickly. You will need to focus on one thing for the next 8 weeks (or whatever) and then focus on the next. This is how you will get the BEST results. You can *try* to do both, but like I said, neither one will likely be done successfully.
If you take AAS (steroids), then itis possible. But that opens up a whole nother can of worms, one which most shouldn't venture into.