I have a thread graft for an elm. The branch diameter of the exiting
portion is more than double the size of the drilled hole. Is it a near
certainty that the graft has taken?

I would assume so.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)