As a general rule: I give Baptists the benefit of the doubt, which is to say that I assume they don’t actually believe what they supposedly believe.
The ‘hard’ Baptist position with regard to infant Baptism (i.e., that it is ineffective) entails that Baptism is a work of man.
As all works of man are ineffective, it necessarily follows from the Baptist position that Baptism is ineffective.
Proposition: Infant Baptism is ineffective.
Corollary: Baptism is a work of man.
Fact: All works of man are ineffective.
Conclusion: Baptism is ineffective.