I think he’s referring to the definition of ambition Quirrel uses in Chapter 70:
The Defense Professor’s fingers idly spun the button, turning it over and over. “Then again, only a very few folk ever do anything interesting with their lives. What does it matter to you if they are mostly witches or mostly wizards, so long as you are not among them? And I suspect you will not be among them, Miss Davis; for although you are ambitious, you have no ambition.”
I think he’s referring to the definition of ambition Quirrel uses in Chapter 70:
I guess it depends on what one finds interesting. Clearly QQ/LV found taking over Magical Britain interesting enough.
The claim is that he found being Lord Voldemort interesting, but he didn’t find taking over Magical Britain interesting.