Killing tree roots

Had a tree surgeon here today taking out a couple of dangerous trees for me. One of them was a willow that had been pollarded many times over the years. Fearful that it may grow again from the stump, I asked him how he prevents this. I was amazed at his answer, he uses undiluted glyphosate poured into cuts he makes between the bark and the wood of the stump. Has anyone else come across this solution? It seems such a simple solution to a common problem

I killed a young tree by using a drip bottle, filled with glyphosate, pushed into a hole I'd drilled into the trunk.

Mum! How could you!!

 

It does make sense to apply it in that outermost layer, which is where the active part of the trunk is, if I remember biology lessons aright.

I used the one that you usually dilute for spraying, but used it undiluted