I think we will never really know for sure, since many alternative (to Xenophon's) sources have been lost, but I believe that Xenophon's role in the Retreat was gradually becoming more important from Cunaxa until Thrace. Maybe he overstate a bit of his own achievements, but not too much.

Xenophon was highly esteemed by the Spartans, under whom he served after the Retreat. They gave him land and honored him by allowing his sons to undergo the Spartan Agoge (they actually died in Mantineia, 362 BC, fighting alongside the Spartan King). My personal opinion is that he can't have overstated too much, minimizing the role of the Spartan Cheirisophos and the other generals, simply because the Spartans wouldn't aproove it. After all, a great proportion of the 10.000 were Spartans or Peloponnesian allies of Sparta, so they must have known the real facts.