I wonder if they knew they were coppicing the tree, rather than killing it. Thankfully the tree will continue to grow, I think the land managers have chosen to leave the tree there. Many of the oldest trees around (at least in the UK) have signs of coppicing and pollarding. Those who cut the tree have unintentionally extended its life (e.g https://www.bbc.co.uk/news/uk-england-gloucestershire-203131...). The only issue is it wasn't done at the best time of year.
You can read more about coppicing on the national coppice federation: https://ncfed.org.uk/public/coppicing/