Probably the same number of arranger players are equally fooled into thinking that they can play piano, too. All evidence to the contrary...
Perhaps, if thinking that one can play an arranger, while one CAN play the piano is an erroneous proposition, the reverse should be a given?
Personally, I think that if one HAS the technique to play a piano well, all that needs to be applied is effort at understanding the operation of the arranger. However, the converse is not necessarily the case...