...it's not Fallacious if it is true, as the last quote explains.
Your quote "explains" no such thing.
It simply asserts that in SOME cases, presumably if the subject at hand is specifically about a person's history and or personal choices, data relevant to such an inquiry CANNOT be considered off-limits.
HOweVer, I can't imagine a case where a person's history and or personal choices would be a subject of scrutiny wholly divorced from any explicit or implicit attack or endorsement of their abilities and or ideas WHOLESALE.
A "positive" ad hominem is just as fallacious as a "negative" ad hominem.
You shouldn't believe someone just because they're a doctor.
You shouldn't disbelieve someone just because they're a commie.
Well that's encouraging.
The only "problem" being that you can never know "their character" or "motivations" etc, due to your EPISTEMOLOGICAL LIMITS.
And even the most morally repulsive, malevolent person can still make a perfectly valid logically sound statement.
Their moral repulsiveness and malevolence do not magically invalidate their logic.
I admire your faith in sample-bias.
Please teach me to "Judge Correctly" without logic.