True.
A person is not considered a heretic for denying a dogma which the infallible Magisterium of the Church has not defined fas such.
That is part of the problem I have with Mr. Drew's Rule of Faith.
The Church has previously defined that Mary is immaculate.
Pope St. Martin I, Lateran Council, 649 A.D., Can. 3- “If anyone does not properly and truly confess in accord with the holy Fathers, that the holy Mother of God and ever Virgin
and immaculate Mary in the earliest of the ages conceived of the Holy Spirit without seed, namely, God the Word Himself specifically and truly, who was born of God the Father before all ages, and that she incorruptibly bore [Him], her virginity remaining indestructible even after His birth, let him be condemned.” (Denzinger 256)