Why is sy used here rather than "mae'r archdderwydd yn arwain..."
Because it is using an emphatic construction. In clumsy English - 'The Archdruid it is who leads...' - to emphasise that it is the Archdruid doing the leading of the ceremonies, not the ice-cream seller or whoever.
More natural English would be "It's the Archdruid who leads...".
Agreed. Sentence in the database now edited. Thank you.