But unlike a lot of movements, it is mostly bottom-up. There weren't ~20M people marching to protest because of some careful top-down orchistration; people were outraged. Black people, white people, Latinos, Asians--everyone was just outraged.

BLM was originally leaderless, and still has minimal formal organization. None of the efforts to run out in front of the movement and declare oneself leader has captured the movement as a whole.

The Black Lives Matter Global Network was (1) started after BLM (though not long after), and (2) to a very modest extent managed to exploit the outrage and reactionary goodwill after the George Floyd incident to gain funds but not a great deal, and (3) has founders who do have an affinity for Marxism but who do not and cannot speak for terribly many of its members.

BLM Global Network had very little to do with the protests and riots after George Floyd. For instance, the Seattle protests--which later turned into the infamous CHAZ occupation--were spearheaded by someone with a wholly separate background from BLM-GN: https://www.newyorker.com/news/dispatch/a-seattle-activists-fight-to-keep-the-focus-on-police-abuse

Any view of the protests in 2020 as anything but populist is incredibly wrong.

BLM Global Network people have tried to cleverly adopt the BLM moniker to advance their own agenda, and they've been doing it enough so they have some pull, but it's still limited. The name makes it sound like they're the whole thing. They apparently have you fooled, and some donors, and I'm not sure how much of the right. They're not completely irrelevant, but they're not the core of the movement. The protests themselves were about as anti-authoritarian as you can get (c.f. Seattle CHAZ--effective anarchy is the exact polar opposite of authoritarianism).

Now, beyond the BLM Global Network, the BLM movement overall does have some problems, none of which you've managed to enunciate. The chief problem is that they're a movement with no coherent agenda. You would think given that ostensibly and in practice #BlackLivesMatter comes up when black lives are apparently shortchanged or undervalued in contrast to non-black lives, and a lot of BLM-supported protests are against police violence, that the key agenda would be reform of the criminal justice system. But, well, no, not really.

If you get into any particular group adopting the BLM moniker--BLM Global Network, for instance; it is the most prominent--then some of your charges apply. But it's important to recognize that the flaws of one group do not apply to the movement as a whole.

And, again, here, the key point is that we were talking about authoritarianism, and the protests themselves were literally the opposite of that. Populism can be in support of authoritarianism (e.g. Jan 6), but it is absolutely necessary in that case for there to be an authority that is being supported.