A central philosophical premise states that prime matter cannot be wholly devoid of form, even if one form gives way to another. Philosophers use this premise in explaining a continuing substrate and successive forms.
But 'this matter cannot exist without some form' is not equivalent to 'this matter never began to exist'. A structural dependence between matter and form does not, by itself, prove the beginninglessness of the structure.
Two issues must therefore be tested separately: the claimed necessary connection between matter and form, and the further claim that their connection is eternal. The latter needs an argument of its own.