How does Socrates go on such a list since all we know of his philosophy is Plato's distillation of it?
Exactly my thoughts and is why I didnt have a place for Socrates on my version of the list.
I suppose some would argue that the early Platonic dialogue's were more Socratic and some academics have attempted to distinguish which period of Platos life particular dialogue's orginate from.
Right, and while there is a good deal of uncertainty involved in this practice, we can see Socrates emerging from this work.
We also have the accounts of Xenophon and Aristotle, though a lot of what Xenophon tells us isnt particularly interesting as he was a rather pedestarian character. Socrates was also in Aristophanes comedy The Clouds though a not particularly favourable impression of Socrates was given
But the fact that so many wrote Socratic dialogs (it was an entire market of literature in ancient Greece) and that Socrates was significant enough to be satirized directly should support the importance
of Socrates - which is what the list is all about.
Socrates' thought comes to us through the work of others, but that, I think, is a strength to his claim as one of the most important philosophers of history. People found his philosophy to be so important that they recorded it for him as best they could and with great effort. That Plato, another great philosopher, so esteemed Socrates that his work is all Socratic dialog is an immense honor.