] We don't know this for sure, do we? He could well have been a Baronet or even a Baron, couldn't he? As Sir Lewis and Lady Catherine de Bourg had no son, we have no sure way of knowing he wasn't a Peer.

We do know that Sir Lewis was not a peer (which would include being a baron). If he were he would be known as Lord de Bourgh rather than as Sir Lewis. He could have been a baronet, but there are reasons to believe he wasn't, the main one being that Lady C, in her confrontation with Eliz., speaks of both Darcy and Anne de Bourgh as coming from "untitled families" on their father's side.