Lore question

The lore that was written before the great purge of lore, is that stuff still canon? Because it brings to question the age of certain champions.

Sion is said to have been in the tomb for centuries and Vlad, Swain, and LeBlanc have been planning Sion's revival for those centuries. But Sion was Urgot's commanding officer before Sion's death, which places Urgot as "centuries" as old as Sion. But then you have Urgot pre-robot-legs capturing Jarvan4 and then cut in half by Garen.

What am I missing?