My father achieved an undergraduate degree from Harvard Extension, does that make me a Harvard Legacy in terms of my application to Harvard College?
No, not at all.
Only the children of Havard College alumni/ae get an “additional look” in the admissions process.
so kids of Harvard PhDs, or MAs, will not count as legacy?
That is correct. They may be considered legacy if they apply to the Graduate School of Arts and Sciences if legacy even counts at the graduate level.
Yes, there can be a legacy advantage for offspring of alumni of a Harvard graduate program applying to that same graduate program. Each graduate and professional school at Harvard has an independent admissions office with its own policies.