I think that you have internalized a version of body positivity that lies on the most extreme end of what is meant by that phrase. Body positivity - be comfortable with who you are and do not put down on others due to their body.
The odds are that I am significantly fatter than you. The odds also favor that I am significantly stronger than you, even if you lift weights. I can also probably walk all day much farther than you can.
Would it be healthier if I lose body fat? Absolutely. Have I tried for 20 years to do that? Yes. I am not ignorant regarding nutrition. I am not lazy. I am not overall lacking willpower. I am fat but otherwise healthy.
Body positively means that my doctor treats my body fat as what it is - one aspect of my overall health. He does not assume that every problem I have is because I am fat, even though changing that would improve some aspects of my health.
Body positively also means that I am not going to hide when I go to the beach. I am going to go shirtless and enjoy myself. If you do not find me sexually attractive, that is fine. If you are going to shame or mock me for my body fat, then you are an asshole. If I catch wind of you mocking me, I will quietly estimate how many times your bodyweight I will deadlift on Monday. If you choose to mock the scars that cover parts of my body from extreme, life-saving surgery, I may feel the need to firmly educate you on exactly what sort of asshole you are.
Body positively often conjures the image of a morbidly obese girl on OnlyFans who lets people pay to watch her binge and intentionally get fatter while she says being purposefully inactive is just as healthy as hitting the gym. The real versions of that person are extremely rare, but their radicalism, vociferous nature, and platform make their voices much louder in comparison. Their argument is also easy to find flaw with and mock, so they get used as if they are a typical example of body positivity.
You are right in that the woman I describe above needs help and is not behaving in a safe or healthy way. I also understand why you might think that is the norm. She is not, though, and I would encourage you to look deeper at the meaning of the "movement."
The "you" above is generic and based on broad assumptions. You, the reader, might be stronger than me and have way more endurance than me. You also might be fatter than I am. The odds are very good that you are also not an asshole. My point was to call out variances from the norm as convenient examples, of which I have plenty in both directions.