Because previously he was adamant that he had not deceived Marion, GGB and JO for financial gain, as that would be a crime.
He was also adamant that all three women contacted HIM, hence he was not culpable.
He was also adamant that his Remakel licence was stolen or lost during 1994 and 1997.
Now he's admitting to placing an ad with the name Remakel, and it described him very differently to who he actually is, and that Marion answered it thinking he was a wealthy and well bred moral man.
Pair that with the fact she disappeared in June 1997 after her last contact/affair with him, that he had her belongings at his house, and her accounts were drained... leads to a very clear plan of deceit for financial gain. IMO.