It is without doubt true that physically some people are stronger than others, and some people are weaker than others.
What Kopp seems to be hinting at is that on the inside we all have access to the same amount of strength and weakness, because we are all connected, are all made of the same substance, and we are all conscious beings.
This brings out the essence of Metta/Loving Kindness meditation, which reminds us that at our essence all beings want the same thing: to be well, to be happy, to be safe, to be peaceful and at ease.
Of course there are things that get in the way of this truth, that mean we feel stronger than others, or weaker than others, or make us want things that actually do not make us happy or safe or well.
For me, I find it helpful to remember the equality of our strength of weakness in two key moments:
The first is when I feel weaker than somebody else - diminished in their presence. The truth is, I have as much strength and worth as that person, and I can embody it in that moment because of that truth.
The second is when I feel stronger and dominating somebody else - proud in their presence. The truth is they have as much strength and worth as me, and I can help them to embody that, and I can remember that the weakness I see in them is also in me.