The premises given in the original post are:
Premise 1 - All B are C
Premise 2 - A is B
Implied conclusion - (A is C)
As such, the syllogism is logically valid, but either or both premises may be rejected as false.
The specific example given is plainly ad hominem, there are elements of poisoning the well and it is a thought terminating cliche. The term apostate is also being used as a euphemism.
But, if the person goes on to claim that they did not say ‘A is C’, they are stupid, lying or both, because they have contradicted their premises. See also weasel words, non-denial denial, spin.