Do you clarify before any fiction story that it isn't real to prevent being seen as lying? There are little ones that believe cartoon characters or characters in books are real without anyone having to tell them they are and many more happy to act like they're real for the fun of the activities around them. Father Christmas is pretty much the same.
We can tell cultural stories and enact rituals around them to give them meaning, pleasure, connection, and interest without saying they're the truth. St George didn't actually slay a dragon, but in years past I've taken my children to town to see it being performed and hear it discussed as an actual event. No one viewed that as lying - it's community spirit and a bit of fun - but every year we have several posts on the lie of Father Christmas as if we aren't surrounded by fiction and various stories and ideals that are debatable in their objective reality.
I mean, when discussing the police, do you tell a young child they're there to protect them - that they're 'helper people' as I often see in books for small kids or do you discuss the history of police as protectors of the wealthy against poor workers and the many cases of police brutality, police cover-ups, and deaths in police custody so you're not lying to them.
I don't do Christmas, so obviously I don't 'lie' about Father Christmas, so I've no skin in that one specifically. Outside when parents pressure their children to still 'believe' for the parent's own idea of how things should be, I can see value in the story and the rituals around it even if I don't take part. We do do tooth pixies for their teeth - their paternal grandmother made them tooth pixies dolls. I never tell them it's real, it's just a fun thing we do, and as they've gotten older we've talked about different myths and rituals around losing teeth from around the world. The representation in Hogfather is one of their favourites, we have the book and they watch the TV version every year and if anything, knowing lots of people enjoy something makes them enjoy it even more knowing it's a bit of fantasy.