While there's been some indication the implementation of this tech has been pushed back a generation, it's existance itself is more than just a rumour; Multi-die Vega/Navi cards have long appeared in AMD slides and some tech demos have reportedly been shown to developers behind closed doors.
Raja has previously said: "It enables some really low latency and high-bandwidth interconnects.This is important to tie together our different IPs (and partner IPs) together efficiently and quickly. It forms the basis of all of our future ASIC designs. We haven't mentioned any multi GPU designs on a single ASIC, like Epyc, but the capability is possible with Infinity Fabric."
It's likely the only reason we've not yet at least seen a traditional dual-die variant of Vega so far(AMD has made at least a "prosumer" version of every top-end chip for many generations without fail, including Polaris) is the same reason we've not seen a Vega Nano: The existing SKUs are (For the time being) already in far higher demand than they expected+can keep up with.
I think we'll certainly see a dual-GPU variant that appears to the OS as a single GPU for Navi at some point, even if only in low volumes and with the full implementation coming in later drivers, to iron out the kinks in the technology as they move to become more dependant on it in the future.
Lets not forget this is technology NVidia has also publicly been researching and planning to use for years.