The platform supports it doesn't mean all the phones do. This is the same on android, BTW. Basically you can expect phones with removable SD cards to support it, those with only internal storage to not (there are exceptions to the latter on android).
As for your link, USB host is in the context of SD cards:
“We introducing removal micro SD support as a core part of the platform and this core micro SD card support spans both the PC and the phone.
And let’s be clear: this mass storage support gives you a PC mountable experience for the micro SD but not access to the phone’s internal memory for security reasons.