European Hornets don't excavate wood out of a tree; they only collect wood fibers to turn into a paper-like material by scraping the surface with their jaws. If holes are present in the tree's bark, they preceded the hornet activity and were created by another insect or Yellow-bellied Sapsucker woodpeckers. You can learn more about hornet behavior on the page linked above. They are likely making use of a source of sugars in oozing sap, honeydew, or slime flux. If so, they are not causing any damage.

Given the size and age of the tree, we recommend you have it evaluated by a certified arborist (or two, as a second opinion). While they cannot treat or cure every ailment, they can help make a diagnosis, as well as look for sources of tree stress that might be able to be alleviated. If hornets are entering and existing a cavity in the tree (since natural cavities are where they build their nest), then a pest control professional (or the arborist) might need to spray the nest entrance with pesticide in order to kill the colony, assuming the area can't be avoided until the nest dies out on its own by winter.

Miri