It was said of him that he painted authentic Vermeers, even though it was known they were not originals. He was (and remains) a controversial figure, but he was undoubtedly an artist.
You can learn more about forgers (and much else besides) in the excellent book "Kunst der Fälscher – Fälscher der Kunst" by Frank Arnau (which I highly recommend).
The art of the faker: three thousand years of deception in art and antiques (English translation by J. Maxwell Brownjohn, published in London)