Acc to prof Chadwick and the links provided by CM, it was Pius X that declared Boniface 7 as an anti-pope. Boniface 6, John 16 and Benedict 10 were also defrocked by St Pius.
The symbol on the back of the chair of Boniface is blasphemous plain and simple. A Pope cannot commit blasphemy and be a True Pope.
A 666 hexagam is blasphemous to a Christian just as a cross would be to a Jew. In case you don't get it yet, the two religions are not the same.