Is it true that enlightenment is equivalent with perfect knowledge? It seems to me that there is a popular and strong belief that if you reach enlightenment, and thus have four immeasurables, you can help a person without making them feel extra suffering or fighting back. This can be illustrated by the sutta Aṅgulimāla. The Buddha can stop him easily and effortlessly. The premise here is that enlightenment = perfect knowledge, and/or four immeasurables = you and the others are truly the same, so that you can understand that person inside out.

The equal sign = is more like implies that rather exactly means.

However, realistically, I think this is impossible. There are situations that achieving enlightenment does not mean perfect knowledge. For example, you can reach enlightenment in a radioactive region. This is not loving-kindness yourself. Likewise, one can never claim to perfectly understand the inner state of the other person, especially without face-to-face conversation. What it seems to be true is that enlightenment = having no distortion, and/or four immeasurables = being able to vision a way to help the others getting out of their suffering.

Perhaps this is just a matter of defining what enlightenment is. This is fine. But in this case, its necessary consequence is that no human can achieve this state (though can absolutely achieve mindfulness or practice eightfold path).

Is this correct?
In buddhism, Enlightenment is the "direct knowledge" about suffering, the origin, the cessation https://www.accesstoinsight.org/tipitaka/sn/sn56/sn56.011.than.html

the four immeasurables  is just how to get to Brahma world, like here https://suttacentral.net/mn99/en/sujato  so it is not Enlightenment. When you claim that the four immeasurables  is Enlightenment you are a hindu and that's totally wrong in buddhism.

So you are mixing hinduism and buddhism. In hinduism the path to nibanna is to generate merits over and over, same thing with the jains. They have to burn up past bad karma and create lots of good karma..
But that's very wrong view.

In buddhism generating merit is useless to get enlightened. Merit gets you a good birth and that's all. In buddhism you need to know about suffering, the origin, the cessation and the path to the cessation of suffering.
So do not confuse the path of merit, the path of Enlightenment and Enlightenment itself.