Ohhhh!!! I see what you are saying now... nope...I thought I had it... but as I was forming a reply it went away..
Are they suggesting that the volcano erupted 485,000 years ago or 300 years ago? Do they know when the volcano erupted? Also, does this suggest that the volcanic ejecta is supposed to start from the zero-rest assumption at eruption? It's not possible that the material ejected may be a bit older than a tree? If the tree was buried 300 years ago, that would seem to me to be quite a large indicator that the eruption happened 300 years ago, yes? Seems like that would make sense to me.