The US has far more experience than China at building large planes when the C-17 came out. This is the first wide body plane that China has build. China will need a longer period to get this plane in full production and in service compared to the US.
By78 merely said the Y-20 may have entered limited service with the Air Force
There is nothing particularly unrealistic or surprising about that. We've known since last year that the first few Y-20s may enter service in 2016.
More importantly, having the Air Force induct a few planes in a limited way is well within their capabilities of the company producing the aircraft because it doesn't require the aircraft to have entered serial production. Cargo aircraft are also still useful in relatively low numbers.
As opposed to fighter aircraft, where one would need to induct a fairly large sized unit like a squadron or regiment to be truly useful, a strategic transporter can be inducted in small numbers like four or even two aircraft initially and still have a variety of practically and training uses given the flexibility of a non combat, large cargo aircraft.
In other words, the idea of a few pre production aircraft entering initial, limited service with the Air Force is actually quite fully realistic, if one understands that this kind of limited service in terms of capabilities is still probably quite far from true IOC, let alone FOC, and also to remember that the aircraft which are in service are probably not true serially produced Y-20s yet.
As for comparison with the C-17; assuming an equal development time for a past US project to a present Chinese project is always fraught with confounding variables. Differences in funding, priority, available technology, may all make such direct comparisons less useful and can either add or subtract the actual number of years needed to complete a project to a certain equivalent stage.
That said, I do believe that XAC will probably take longer to successfully mass produce a number of Y-20s because this is their first go at mass producing an aircraft like this and mass production and quality control during mass production is where the problems tend to crop up... but at the same time I think it is conceivable for them to have finished most of the flight testing and development within three or so years and for the initial pre production Y-20s to be handed over to the Air Force for initial service.