The "as ... as" construction does not work with a noun as the first argument.
We could say "They're not as trustworthy as we think they are." The adjective "trustworthy" fits this construction nicely. We could even say "They're not as trustworthy and friendly as we think they are." -- the compound adjective phrase also fits nicely. We could even compound the complete first part of the "as ... as" construction: "They're not as trustworthy and as friendly as we think they are."
However, the speaker didn't use the adjective "friendly". He used the noun "friends".
Even though it makes sense to say "They are not as friendly as we think they are", it does not make much sense to say "They are not as friends as we think they are."1 When "as" takes a noun as its argument, it means something different and it stops working as the start of an "as ... as" construction.
If we really wanted to, we could keep the second "as": "They are not friends as we think they are." However, this can imply that they are friends -- merely friends of a different nature than we understand.
Let's not forget that we've learned something. We used to think that our minds and feelings were our friends. We don't think so today. The past tense in the original marks that we've changed our minds. We no longer think what we once thought. Wouldn't it be foolish to keep making the same mistake, even after we've already learned better?
Perhaps our minds and feelings are still our friends. We were wrong in our thinking yesterday, and it's possible we're wrong about different things today. Perhaps our minds and feelings are still even trustworthy in some fashion. Even if so, they are not the same as the trustworthy friends that we used to imagine them to be.
Yesterday, we thought that they were trustworthy friends. We changed our minds. Today, we probably think that they are unreliable acquaintances.
1 The sentence "They are not, as friends, as we think they are" does make some sense. However, it means that they are friends -- they merely serve in that capacity differently than we expect. This is not the speaker's intended meaning. He simply denies that they are what we mistook them to be in the past.