The argument is that by taking animal dung and coating the entrance they are "taking something and manipulating it to shape their environment", which they think qualifies as tool use.
But we've already known for ages that bees gather sap from trees and coat the inside of their hives with it (propolis), to seal gaps and prevent bacterial growth. I cannot think of any reasonable definition of tool use that would include dung coating but exclude sap coating.
Ctrl+f of "sap" and "propolis" in the original paper gave 0 results.
Edit: Upon further reading, they seem to define a tool as a "a non-plant solid", which seems specifically tailored to make dung the first tool. The fact that they fail to mention propolis even once in the paper to highlight what they think is the important difference does not impart confidence.