Some of the English in the part that you've highlighted is anachronistic, and this is a hard sentence for even a very fluent reader. "Answerable for" means "responsible for", "surmise", in this case, is a noun synonymous with "guess", and "just" is synonymous with "accurate". Essentially, this part of the sentence means: "Darcy was, in truth, just as responsible for Wickhams absence as he would have been if Elizabeth's first suspicion had been correct."
So what was Elizabeth's "first surmise"? You need the context of the paragraph which immediately precedes this one:
TILL Elizabeth entered the drawing-room at Netherfield and looked in vain for Mr. Wickham among the cluster of red coats there assembled, a doubt of his being present had never occurred to her. The certainty of meeting him had not been checked by any of those recollections that might not unreasonably have alarmed her. She had dressed with more than usual care, and prepared in the highest spirits for the conquest of all that remained unsubdued of his heart, trusting that it was not more than might be won in the course of the evening. But in an instant arose the dreadful suspicion of his being purposely omitted for Mr. Darcy's pleasure in the Bingleys' invitation to the officers; and though this was not exactly the case, the absolute fact of his absence was pronounced by his friend Mr. Denny, to whom Lydia eagerly applied, and who told them that Wickham had been obliged to go to town on business the day before, and was not yet returned; adding, with a significant smile...
So, when Elizabeth came and didn't find Wickham, she thought it was because Darcy had made sure that Wickham wasn't invited. In truth, Wickham chose not to come, because he was the one who didn't want to be around Darcy. In either case, however, Wickham's absence is caused by Darcy's presence. For this reason, Elizabeth is disgusted with Darcy.