I'm surprised they were able to save even some of that tree. Velpar [1] is a mean herbicide that can move in the soil. I haven't had a job where I was required to use it, but am quite aware how nasty it is due to what occurred in a picturesque valley that had a line of nice looking hills for background. The boundary lines for some of the rural blocks in our part of the world follow along the top of hill ridges and typically fenced, and often with the main access track which can become overgrown if not regularly maintained. One year (about 15 years ago) someone had a really good idea that velpar would be ideal to clean up around the fence track and sprayed their top hill boundary a little bit too enthusiastically to put it mildly - no doubt for some period the result was a nice clean track and new saplings growing too close to the fence would have died promptly ... but then like a cancer as the herbicide moved downwards, people started to notice a line of dead trees getting ever wider as the months went on. There's dry seasons so for a time it didn't stick out like a sore thumb, but certainly a time arrived where the line of green hills was interrupted by a block sporting nothing but dead trees on it - it was quite easy to see this 15 miles away (approx 25 km)
[1] https://en.wikipedia.org/wiki/Velpar