The Unified Theory skill feat states that:
Whenever you use an action or a skill feat that requires a Nature, Occultism, or Religion check, depending on the magic tradition, you can use Arcana instead.
The Natural Medicine skill feat states that:
You can use Nature instead of Medicine to Treat Wounds.
One interpretation is using Arcana in place of Nature check to Treat Wounds, because it is a skill feat that requires a Nature check.
Another interpretation is requiring the action's or skill feat's Nature check to have something to do with the Primal tradition of magic, since there is line "depending on the magic tradition".
Finally, there are traits for "Arcane" "Divine" "Occult" "Divine" and "Magical". If they really wanted to limit the Unified Theory skill feat to only work in those specific cases, there were ways to do that clearly and mechanically.
How would you run it? Is there a definite answer?