Mac user and future Ridgeline owner here...
When you say it played an MP3 CD, that could mean two different things.
First, it's possible to use MP3 files to burn a "normal" music CD. Toast on the Mac can do this. It takes MP3 song files and converts them to "normal" song files (AIFF format I believe) resulting in a everyday CD that most any modern CD player can play. Due to file size after conversion, the CD can only hold about 12-18 songs, depending on length.
Second, you can burn a "data" CD featuring mp3 files. In this process the mp3 files remain mp3 files, allowing you to burn 200-odd mp3 files onto a single disc. This disc can NOT be played on regular CD players. It can only be read by a computer or by a player built read to an MP3 data disc.
So unless they designed the 6-disc player to play MP3 data files, it's not an MP3 player. Since they sell another device for this purpose, I would find it very surprising to learn that it does play MP3s.
Scott