The only source of the claim that the existence of.linux/landlock.h is insufficient is (AFAICT) the malicious git commit. Why trust the comment, written by the attacker, to explain away a malicious change?
I already explained above why the existence of linux/landlock.h is not sufficient. Why do you still question it? If you know a bit about system programming and how configure checks work, the change in itself is totally reasonable.
