I come across ground wires put together like this constantly.. I always at minimum use a crimp (I also placed a crimp here after the pic)
It doesn't look the wires in the pic were twisted too tightly. I can see between them. 110.14(B) says splices are to be made with devices identified for the use
I think a lot of amateurs and professionals alike think "The ground wire doesn't ever do anything, so there is no need for a connector."
Around here, the crimp barrels are pretty popular, but they are often "crimped" incorrectly, that is, just flattened onto the twisted conductors with pliers making for a less than ideal connection.
Not enough sheath in the box?
Where the ground is crimped must extend 6 inches out of the box
That is: 6 inches of free conductor in the box, 3 inches outside the box. (300.14)