why would rebuild index reduce btree leaf numbers
A living index often accumulates empty spaces in the leaf nodes — e.g. due to deletes. Rebuilding the index will make sure all leafs are filled up to the configured threshold.

Note that I wrote it might reduce the number of leaf nodes. If have only inserts on this table, you won't even get this benefit.

Also keep in mind that the overall context of this article is an argument against rebuilding indexes without really good reason.