Hands on heart I did not take the example sentences from there. Besides, I cannot find them there.
So am I.
A gourd was cracked open. -- In this sentence the copula is 'was' in my opinion. What is 'cracked open' then? 'cracked open' is an adjectival idiom which has unitary meaning.
(these ideas were not taken from CGEL)
The sun shone. -- not a copular use of shone
The sun shone bright -- copular use to describe the sun
The sun shone brightly -- no copular use; brightly describes action
You agree?