It's a reciprocal verb, and has its own entry in the dictionary separate from 'å se'.
Reciprocal verbs describe something that two or more people do to or with each other, and always take either a plural subject or two or more singular subjects. There's an implied 'hverandre' (each other) in the verb.
Seeing as English lacks a reciprocal verb to translate 'sees' to, I'll use 'møtes' as an example instead:
"Vi møtes ofte."
"We meet often."
There is still the option to use a non-reciprocal verb, and add 'hverandre' to get the same meaning:
"Vi møter hverandre ofte."
"We meet each other often."
I agree, the explanation looks slightly misleading at first glance. I think the point is that "se(e)s", when you first encounter it in a sentence, could represent either the passive or the reciprocal form, and it's up to us to determine which. The person who commented was asking why "vi sees" can't mean 'we see', and Deliciae was giving them the two options as to how it could possibly be translated into English: as passive, and the set idiomatic phrase. Possibly she focused on the passive because most of the rest of the comments on this page are already about the reciprocal form. :-)