Like Richard Stanley, I also did not work out the details. I also can't say if you can prove it from Sperner's theorem. Here is why I think there is no larger $k$.

Split the bipartite graph into two based on whether they contain the element 1 or not. You will see the collection of n+1 sets fanout into the n-sets when they don't have the element 1, and the fanout goes the other way for the sets that do have 1. Your current solution shows the sets at the narrow end of each fanout. Suppose you want to improve upon it.

This means you will have to take (say) some sets from the n+1 camp, and replace them with even more sets in the other camp (and likewise for the n sets). But however you arrange the n+1 sets in the other camp, if they are all contained in a subset of size less than 2n+1, they will fan out to cover more subsets of size n than they replace, so whatever you choose has to cover the whole set of size 2n+1.

So now you have to pick j many subsets from 2n+1 elements of size n+1 that cover exactly j subsets of size n: this means each subset of size n has to be covered by n+1 subsets, which is all possible covers of that n set. This means any for any n set covered by one of your j cboices, you also need to cover another n set formed from this n set J by swapping any one element in J for another not in J. But now this closure condition means your j elements have to cover all n sets. That's too many. So it can't be done.

Gerhard "Look Ma, I Can Fly!" Paseman, 2019.05.22.