FEZ has Arduino-compatible libraries?
Why of course not ... I never said the FEZ had Arduino compatible libraries but it has the same form factor as the Arduino. 
What the FEZ brings to the table is several years of proven hardware development in which I2C, SPI, Serial communications just plain works.
The Maple is "Arduino compatible" but it seems that Leaflabs have their guru hardware programmer working partime to finish the Maple development project that should have been finished months ago.