Since you don't know seem to know much about how engineering works, let me explain. When there is a project reveal, that typically means the engineering is complete. There is no further development required, which for a train system would be the locomotive, tracks and signalling are all completed since they come as a package. What is probably not done is the procurement for land to put down the tracks and construction.5-10 years to build a train line? taking into account how fast chinese companies work, and the fact that only now the train was unveiled, i doubt that. I think that the train/track are still in development phase.
The train is 600 km/h. Calling it "first 600kmh train" could have been a public statement lapse/mistake.
So when people say 5-10 years to completion, all that is left is building the line and testing it.
They can say first km/h line because it would be the first commercial line. Japan's maglev is only meant to go commercially at 500 km/h if it ever does get built, but who knows when.
Also, announced speeds are operational speeds, and not max design speeds. So Japan's 603 km/h max speed which the system is designed for is the same as China's announced operation speed. The Chinese design/max speed could be 100 km/h more at 700 km/h, we won't really know until they test it.