Wi-fi chipset with SDIO interface in SPI bus mode
After some more research I came accross the answer in an extension part of the SDIO specifications.
In section 1.1.2 of Part E1 Simplified - SDIO Simplified Specification V3.00 embedded SDIO Devices such as Wifi chips with SDIO interface are mentioned.  

There it says that the SPI mode is optional for embedded SDIO devices.