Я уже привёл выше статью, где явно говорится о том, что это не так:

"The leaves of the btree contain items, which are a combination of keys and data. Similarly to reiserfs, the items and data are packed in extremely space-efficient way: the item headers (that is, the item structure described above) are packed together starting at the beginning of the block, and the data associated with each item is packed together starting at the end of the block. So item headers and data grow towards each other, as shown in the diagram to the right."

Твой черёд доказывать свой тезис.