What is the grammatical function of "circumstances as they are" in this sentence...

I find this sentence in an online article strange and even jarring for several reasons.

With limited provisions, he awaits reinforcements, but circumstances as they are, is believed lost to the wilds. (source)

I am not quite sure what grammar function "circumstances as they are" takes. It seems there's missing components. I have never seen things as they are or circumstances as they are stand alone in a sentence like this (except in book titles). I would have written:

With limited provisions, he awaits reinforcements, but given circumstances as they are, is believed lost to the wilds.

Am I correct in thinking the sentence is ungrammatical as it stands? If I am wrong, what grammatical role does "circumstances as they are" take? Is it adverbial?

I believe it’s technically a prepositional phrase. There’s an understood “with” here: “...he awaits reinforcements, but with circumstances as they are...” That’s just one of those technically-improper-if-you-want-to-split-hairs-but-people-say-it-like-that-anyway phrases. With your edit, the “given” seems to be acting as an adjective and doesn’t affect the grammatical structure. Either the original or way is fine.