It is not an unreasonable claim. Of course it's a bit jarring to see it stated in such a matter of fact way, when we are used to hearing that everything related

It is not an unreasonable claim. Of course it's a bit jarring to see it stated in such a matter of fact way, when we are used to hearing that everything related to art is "subjective". But if we look at somewhat objective criteria like influence on other artists and his range of different styles and media, he is probably unsurpassed in the 20th century. And then his productive years nicely overlaps with 3/4 of the century (during which he kept reinventing himself), while many other artists are associated with a single period or movement. Of course that doesn't mean that he has to be your favorite artist.