Corner cases in the Interleave Lower Bound for BSTs
Leaves have no interleaving since they only have left sub-trees. Therefore you should apply the bound to all non-leaves, in which case the lemmas in the appendix go through. The resulting lower bound should be exactly the same.

The paper in question appeared first in a conference (FOCS) and then in a special issue dedicated to papers from the conference. Usually the conference reviewers are recruited to do the journal reviews as well, in the case of a special issue. This lowers the standard for reviews, since conference reviews are often not very exacting, and when reviewing the same paper for the journal, the reviewers won't bother to read the paper again. Exacerbating this phenomenon is the fact that the lower bound appears in an appendix.

The experts probably realized this inaccuracy right away, and knew that it wasn't a real problem, so didn't bother to comment about it if they noticed it at all.