No, he did not know her fully.
In fact, his feelings had very probably detected her true nature, hence his proofs of confidence. But he had not aknowledged them. Probably because he had never asked himself.
My opinion is that Darcy had never met yet, or never been interested in, any woman like Elizabeth. Everywhere he had met with mediocrity of feelings in women. And he was looking for absolute, but had no idea yet of what it was, as shown in his description of an accomplished woman at Netherfield in chapter 8.
When he discovers ELizabeth, he feels she is the one, before aknowledging, and if he is so slow in his aknowledgement, it may be because he had not analyzed what he wanted.
Their "explanation", or rather his recollecting and understanding it, was certainly a revelation to him, as it and his letter, were so to ELizabeth, to whom it was not enough to love him because she did not love him before.
And his unjust accusation was to me, a burst of anger showing how much he was out of himself, rather than a well thought belief.