В мануале и в хелпе написано:
q, --quiet
    Exit non-zero for improperly formatted checksum lines
--strict
    Don’t print OK for each successfully verified file
а по факту выполняется наоборот:
$ cat summ.xxh64
8bd__broken_hash__  test.txt
1192995662e0804d  test1.txt
69f55c449b3001a6  test2.txt

$ xxhsum --strict -c summ.xxh64; echo "exit status $?"
test1.txt: OK
test2.txt: OK
1 line are improperly formatted
exit status 1

$ xxhsum --quiet -c summ.xxh64; echo "exit status $?"
1 line are improperly formatted
exit status 0