And also perhaps a subtle 'zing' at Caroline Bingley? (Interesting that CB doesn't mention reading in her catalogue of female virtue!) I suppose she doesn't read much!

If Lizzy takes the comment as an insult, it is perhaps because Darcy agrees with all CB says ("All this she must possess") and adds to it. That *is* quite a lot for a twenty-something to have achieved. (And perhaps she might also have a degree in rocket science?)