Dual-licensing in that sense is far more common with GPL'd apps/libraries where the copyright holder offers a separate commercial license for the same code.
The Fuchsia source is primarily BSD licensed (with some MIT bits like the Magenta kernel, and some third party code generally of the MIT/BSD/Apache2/Zlib varieties).
Unless I'm mistaken dual licensing GPL'd code also requires holding the initial copyright. It also adds complications around outside contributions. With BSD they can accept contributions under that license and still license it themselves.
The Fuchsia source is primarily BSD licensed (with some MIT bits like the Magenta kernel, and some third party code generally of the MIT/BSD/Apache2/Zlib varieties).