How come here this bash_completion can perform it

[Budi]

How can, here, the bash completion can work something when it has none, as

complete -p

shows so ?

how can one have it that too in order to append it to his own COMPREPLY

So confusing
has tried this to fail:

complete  -D -F dir_test m

$ complete -p
complete -F _dir_test -D

not only why - move or where is -D from, but even where or how the
target m get nullified

Please help out!