Are the blue and green forces in the diagram below an action/reaction pair?
Yes. The Blue and Green forces depict the pair described by Newton's Third law.
If so, how does the process of equilibration described in the text make sense?
Keep in mind that the process described in the text is occurring at the molecular level. Specifically, the website states that "the entire system very quickly finds its balance, and since the wall and box are very stiff, the final displacement isn’t perceptible."
The back and forth (equilibriation) motion that is being described is not actually perceptible. While this is not incorrect, it is also not incredibly useful to understand third law pairs in this way which seems to be the reason for your question in the first place.
For the sake of completeness, let's look at the other pairs as well:
The Red and Green forces are both acting on the same object. These cannot form a Newton's Third Law pair because the Third Law requires the forces to be acting on two different objects.
The Red and Blue forces do happen to be acting on different objects but are not acting in opposite directions (the other requirement of Newton's Third Law).