The chapter makes its case from two observations anybody can check, one about people and one about plants. A person is soft and weak when alive and firm and strong when dead. Grass and trees are soft and tender when alive, and dry and withered when dead.
From those it draws a general classification: the firm and strong are the companions of death, and the soft and weak the companions of life.
The move is an inference from a correlation and the chapter knows it is doing that, since it supplies two independent instances before generalising. It is also the plainest statement of a preference the book asserts everywhere else without argument. Chapter forty says weakness is how the way works. Chapter forty-three says the softest thing rides over the hardest. Chapter seventy-eight says nothing is softer than water and nothing surpasses it against the hard. This chapter is where the preference is given a reason.
The reason offered is not that softness wins contests. It is that softness is what living tissue is like, and rigidity is what a corpse is like, so the qualities are diagnostic of a state rather than instrumental. Strength is not being called ineffective; it is being called a symptom.
The chapter then applies the classification twice. An army that is strong will not win, and a tree that is strong will be cut down. Both applications are about being singled out: a formidable army invites coalition and a good tree invites the axe, which is the argument the Zhuangzi builds at length around useless timber.