Treble attempts to fix fragmentation by re-standardizing and more loosely coupling between the driver layer and system APIs. We won't know if this is a solution until later and this XKCD[0] kind of explains why.
This is a real time OS from the ground up. Some have SPECULATED it might be run in a VM/container rather than directly on hardware, effectively further standardizing in a similar way to Treble but via a virtual machine's interface instead of a series of system APIs.
That would allow Google to ship Fuchsia VM/container images that could run in every single handset no matter the hardware, and therefore every Android device could be updated directly by Google at the same time.
Meaning Fuchsia may not replace Linux, it might run above Linux and therefore above Treble. That's why the two aren't mutually exclusive.
PS - This is how Microsoft's XBox One platform is engineered currently.
