No, custom soundtracks is where a game properly integrates your music into the game. Meaning if we take Need for Speed: Hot Pursuit for example, it allowed you to add your music into the game. It wasn't just simply using the console music player to just play music over the game, but NFS let you pick what songs you wanted to use, where you wanted to use them (do you want this song to play during races or only in the menus?), and properly transitioned when going between racing and back to the main menu.
Examples of proper custom soundtracks would be:
GTA games on PC and on OG Xbox
Need for Speed: Hot Pursuit (2010)
SSX (2012)
there are others, but the point is simply playing music over a game via the console player is not the same as actual custom soundtracks. Hopefully my description made some sense.