Well, the whole point that Thucydides is making when he brings these to guys up for the first time is that their story was the dominant narrative Athenians told themselves about expelling the tyrants, even though that narrative is patently false. Th. addresses the tyrannicides at two points: 1.20 and 6.54ff.

1.20:

Such then I find to have been the state of things past, hard to be believed, though one produce proof for every particular thereof. For men receive the report of things, though of their own country if done before their own time, all alike, from one as from another, without examination. For the vulgar sort of Athenians think that Hipparchus was the tyrant, and slain by Harmodius and Aristogeiton, and know not that Hippias had the government, as being the eldest son of Pisistratus, and that Hipparchus and Thessalus were his brethren; and that Harmodius and Aristogeiton, suspecting that some of their accomplices had that day and at that instant discovered unto Hippias somewhat of their treason, did forbear Hippias as a man forewarned; and desirous to effect somewhat, though with danger, before they should be apprehended, lighting on Hipparchus slew him near the temple called Leocorium, whilst he was setting forth the Panathenaical show.

That's the story in brief. 6.54ff has the fuller version of the story, not worth posting here, but I'll link it. The interesting thing there is that Th. presents evidence against the Athenian version of the tyrannicides, specifically the existence of an inscribed altar:

And amongst many that had the annual office of archon, Peisistratus also had it, the son of Hippias, of the same name with his grandfather, who also, when he was archon, dedicated the altar of the twelve gods in the market place and that other in the temple of Apollo Pythius. [7] And though the people of Athens, amplifying afterwards that altar which was in the market place, thereby defaced the inscription; yet that upon the altar that is in the temple of Apollo Pythius is to be seen still, though in letters somewhat obscure, in these words:

"Peisistratos son of Hippais set up this monument of his archonship in the precinct of Apollo Pythias."

I cheated there a little and used Lattimore's translation of the inscription instead of the one linked at Perseus, because Perseus' is unreadable archaic English.

Now that altar actually survives, and supports Th.'s account.

The myth of the tyrannicides was very compelling, however. The Athenians set up statues to them in the Agora, which survive in a Roman copy. They commemorated it in their coins (lower right if you're having trouble finding it).

What you're looking at here is national myth making. For whatever reason it wasn't cool for Athenians to believe the Spartans threw out the tyrants; they needed to have done it themselves, and that's the story they told despite the evidence. Consider the popular American myth of the First Thanksgiving, which /u/anthropology_nerd addressed here.

However false these national myths are, the fact is they have a huge impact on our understandings of ourselves.