Ah ok this makes it clear it is not Engels who has a misunderstanding but you.

But surely everyone can tell the difference between, say, a kingdom, and collective decisionmaking.

Obviously but the difference is not that of authority but of form. The collective decision will be just as authoritarian on the minority who disagree as the kingdom is to those not integrated into the power structure.

should sooner listen to the anarchist arguments, which also criticize the inherent authoritarianism in capitalism. One doesn’t even need to be an anarchist to engage with that.

The anarchist definition of hierarchy (and thus authority) constantly shifts to accommodate whatever hierarchy anarchists currently want to defend as “not really hierarchy” or “justified authority” or “expertise” or “coordination.” Parent and child, doctor and patient, teacher and student, elected militia command, workplace coordination, revolutionary defense, public health measures, collective discipline: suddenly these are not hierarchy, because even anarchists know society cannot function without structured authority.

Authority, in the sense in which the word is used here, means: the imposition of the will of another upon ours; on the other hand, authority presupposes subordination. Now, since these two words sound bad, and the relationship which they represent is disagreeable to the subordinated party, the question is to ascertain whether there is any way of dispensing with it, whether — given the conditions of present-day society — we could not create another social system, in which this authority would be given no scope any longer, and would consequently have to disappear.

Sounds like Engels does have a good grasp

does the crew agree collectively to pick such a leader, or is the leader forced upon them by threat of violence?

That's related to, but not fundamental to the definition of authority, whose core point is enforcement via class violence (IE either capitalist authority or proletarian authority). You can have a completely democratic selection process, and the enforcement of that decision is still "authoritarian", unless you want to allow everyone to break the decisions that are reached.

Marxists already have this collective democratic process, its called democratic centralism, which takes many different forms depending on the situation, but is essentially "diversity in discussion, unity in action". One of the first practictioners of it was Mao, who impressed that individual red army units during the Chinese civil war should select their own officers.

Once decisions are selected, they are binding upon the members. I can tell you from experience that "anti-authoritarian" types consider even democratic rulings "oppressive", and that they feel free to break them since that imposes on their individual freedom.

What if they want a different leader, are they able to choose a new one?

That also is related to, but not fundamental, to the question of authority. Recall exists(ed) not only in communist countries, but even in liberal dictatorships, where recall does absolutely nothing to hinder the authority of the capitalist class.

If anti-authoritarianism to you (like many anarchists) means the freedom to disobey (even democratic rulings), then its no wonder that every single historical anarchist attempt has lasted less long than it took most of us to get through highschool.

The only way for a revolution to survive is for proletarian authority to be even more organized and more disciplined than its capitalist opponents.