That information is false.
iTunes can play audio formats MP3, AAC, AIFF, WAV, and Audible aa, all can be created with 3rd party software. There is also iTunes audio formats like m4a (AAC), m4b (AAC), m4r (AAC), m4p (encrypted AAC) and Apple Lossless.
If the MP3 is encrypted for another player, iTunes cannot play it. Just like the reverse, iTunes encrypted AAC files cannot be played by other AAC players.