See www.m-w.com. Note definition 6 for as, the conjunction.
Our understanding of nature, incomplete as it is, = Our understanding of nature, incomplete though it is, = Our understanding of nature, though our understanding of nature is incomplete,
Main Entry:
2as
Function:
conjunction Date:12th century
1
: as if <looks as he had seen a ghost — S. T. Coleridge>
2
: in or to the same degree in which <soft as silk> —usually used as a correlative after an adjective or adverb modified by adverbial as or so<as cool as a cucumber>
3
: in the way or manner that <do as I do>
4
: in accordance with what or the way in which <quite good as boys go>
5
: while, when <spilled the milk as she got up>
6
: regardless of the degree to which : though <improbable as it seems, it's true>
7
: for the reason that : because, since <stayed home as she had no car>
8
: that the result is <so clearly guilty as to leave no doubt>
CJ