But it's a pretty good indication. Since the firing of spark plugs in a distributor based ignition system is controlled by the camshaft position, any change in timing will be because of a change in the cam timing.
Also, for proper timing the distributor should be almost exactly centered in its adjustment range. Any difference and you should suspect a valve timing issue.
Obviously, the above doesn't apply for distributorless ignition systems. Or vehicles (like American cars for the last umpteen years) that use a crankshaft sensor.