Being left handed doesn't impede progress learning piano using the DecPlay method. Both hands are playing patterns at the same time and whilst being right handed might slightly speed up the process of learning the right hand part, being left handed will have a similar benefit with the left hand part. 

In the beginners style, chords are played in the left hand. In more advanced styles (covered in the Piano Bundle), chords are played in the right hand (with or without the melody) and the left hand plays a 'bass part'.