access method: are disk pages mandatory?

in bufpage.h:

"all blocks written out by an access method must be disk pages"

but in
http://www.postgresql.org/docs/8.4/static/storage-page-layout.html

"Actually, index access methods need not use this page format. All the
existing index methods do use this basic format, but the data kept on
index metapages usually doesn't follow the item layout rules."

I'm not getting it: am I supposed to use the "disk page format" when
writing an index access method, or it's just a "good practice" because
it makes the handling easier? Given the docs it looks "recommended",
but the comment on the code sounds more "mandatory".