The life-span of an artist is typically dealt with by historians, critics, and biographers as a linear process of artistic, conceptual and stylistic development.  Picasso is known, as the statement here indicates, for his multiple styles, which could run concurrently.  True, there are the well-known blue periods, pink periods, primitive periods , cubist periods, etc.  Picasso did not see any of these as developing from one to the next.  He did not conclude one to embark on a new one.  They emerged at will---early, late, early, middle, late, early---from the well of his essential nature. He saw himself as sheer force. 'I am that I am' is also one of the names of God, and Picasso, in the context of art, understood, and lived,  the nature of pure creativity, a power that does not start in a limited way, then gets stronger, or better, as it matures, but a power that is always full no matter what the form of its manifestation.

I don't see a grammatical problem in his protest.  The philosophical difference between 'I am' and 'I [don't] develop' is the classical 'Being versus Becoming'.