That's a very important "largely" in that first description.
Although a responsible scientist prefers more specific numbers to
weasel words like that, I think one probably could quantify this
better by anayzing how twins raised apart respond to questions
about their emotional reactions. My own guess, absent any such
data, is that the "largely" is over 80%, though you are correct
that 100%--the "tabula rasa" idea--is clearly false based on
overwhelming evidence.
I know from my own experience that I can train my own emotional
responses to a large degree. How else could I get angry at Statist
and Theist propaganda when I was raised a Baptist and Democrat?
It would be just as silly to take either of the extreme positions
of this debate as it is any other nature/nurture debate. The only
questions are percentages, consequences, and techniques.