The first part of the description - date of birth / graduation, Slytherin, disappeared in Albania - was meant to make us think Bones figured out that Quirrell = Riddle. Shock! Then nothing after that tracks, and apparently she's talking about a good guy? Huh?

So, apparently what happened is that Voldemort waylaid one of his (usefully Noble) classmates in Albania early on, then decided to assume his identity part-time twenty five years later, with the goal of rising to power as the hero who vanquished the Dark Lord (who he also was). Bones thinks that Quirrell's Yule speech about a 'Mark of Britain' sounded familiar; so apparently he wasn't able to give up on the tyranny bit. I think we can take Quirrell's explanation to Hermione as roughly similar to fact; he saw that the hero wasn't getting respect, and people were falling all over themselves to grovel before the Dark Lord, and decided to stick with the persona he found more pleasant to assume.