Welcome.
Playing a ritardando involves a gradual decrease in the tempo of the piece of music you play.
Yes it can be achieved if you are accompanying yourself or playing along with a backing track that includes it.
Style playback involves playing along at the tempo you set and the tempo stays the same until you stop playing, it is just a feature of the set auto accompaniment that is a function of these DP’s and of Arrangers. So no, you cannot play ritardando under these conditions since the Style playback tempo is fixed.
I believe it may be possible via MIDI control of the tempo via a pedal but I am unsure if your DGX is capable of this level of control. Or tap tempo controllers are available but I am unsure if they would work with your DGX.
OK so that said it is worth having a play with your keyboard to see if by using the Tap Tempo button on the keyboard whilst playing you can achieve a slowdown of the style playback, can you try this and get back to us?