Because anarchy is not the question of reparative and rehabilitative justice, that's cop talk. Justice is a liberal proposition, if you want to do anarchy, you must think bigger not liberal+.

If you want to talk justice, constitution, law, etc you can join your local social democracy party. Just stop calling yourself anarchist, stop diluting words.

literally what he describes earlier in that video…

And then he says "they should lock people up".

wait, you’re not anarchist?

I am not partial to the crime or wrongdoing in question. If I am, then I will have an opinion. My point is that prescribing solutions from the outside of a situation is authoritarian. Drawing up a constitution and a model for totally-not-prisons (institutions that will perpetuate themselves at the cost of solving the problems they were built to solve) is authoritarian.

Literally imprisonment.

If you think that shaming, ostracism, exclusion and exile are forms of imprisonment (which is different from prison as an institution, but we'll get to that) , I don't know what to tell you. Sure, shaming and similar rites probably involve detainment like stocks and stuff, but the timescales involved are way shorter and the purpose is shame, not detainment. Also why did you bold ADR?

The institution is the guardians, the physical cages, the perpetrators, and the victims.

so like a third party doing this, instead of the folks affected by the wrong doing?

You don't know what institutions are? That makes sense. An institution is an established custom, practice, relationship, organization, or behavioral pattern of importance in the life of a community or society. Marriage is an institution. Prison is an institution. They are frameworks that exist independently of the people flowing through them or the physical infrastructure facilitating them. They grow in complexity in response to new problems (like more paperwork for a marriage and organizations establishing new departments, see the organizational charts of any government from a hundred years ago vs. today), they perpetuate themselves and calcify. They are the machines that anarchists rage against. The first sentence of the wikipedia page on anarchism:

Anarchism is a mostly far-left political philosophy and movement that seeks to abolish all institutions that perpetuate authority, coercion, or hierarchy...

A prison (no matter how rehabilitative or how well managed with democratic oversight) is still an institution that perpetuates authority (of the guards over the inmates for example), and quite obviously coercion. Do you see the contradiction of trying to formulate an institution that enacts coercion in a supposedly anarchistic system?

Ostracism and exile for example are much more compatible with anarchism as forms of discouraging bad behavior, because they only limit the positive freedoms of the person. No coercion needed when the community just refuses to engage with the person until they right their wrongs.