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STM32F4 MMC compatibility: 4.2 or 4.1?

Question asked by konz.ned on Feb 21, 2012
In the reference manual (section 26.1) it says:

The SDIO features include the following:
ā— Full compliance with MultiMediaCard System Specification Version 4.2.
Card support for three different databus modes: 1-bit (default), 4-bit and 8-bit

but later, in Note 2, we see the odd sentence:

The current version of the SDIO supports only one SD/SDIO/MMC4.2 card at any one time and a stack of MMC4.1 or previous.

Why does it specify "a stack of MMC4.1 or previous"?

There were some major changes between the MMC4.1 and MMC4.2 specifications; one very important one was that 4.2 added the ability to specify addresses using 32-bit sector numbers instead of 32-bit byte addresses.

Can someone explain how the SDIO is fully compliant with the MMC spec 4.2 but not with an MMC 4.2 stack?