What confused you exactly ?

I could be wrong, but if I say "It is not the case that A implies B", then I cannot make the logical claim (A implies not B), that would not be a valid inference. If I deny the material implication (conditional) of (A > B). Then all I know is that A and not B. It does not have to be true that when A happens the negation of B does.

When we negate the particular statement of (Some A are B), we affirm that there is not a single A that is a B. If is is impossible that there is not at least one A that is a B, then we can infer that No A are B, which he expresses as A > B