Thing is what is called start to build and on full scale without being detected?
Almost is certain that China has been preparing the sub systems all over the places for the new carriers for some time now. Can Satellites see them?
Modern shipbuilding is all about modular, working in separate phases and/or locations with smaller parts before jointing them together in a giant shipyard. Once its jointed as a big ship, like any sat can testify, the rest of the process can be relatively very short, a year or even less perhaps, as long as all sub systems are ready for installations.
If China says it has no intention to build, or hasnt started to build, a second AC etc, it may well means it hasnt reached the point where different parts are required to joint together yet Chinas diplomatic definition of whats start to build or on full scale.
Either way, waiting for several years to build one until having learned enough from lessons of Liaoning is not a good reason IMO, since it was an old Soviet design which can be improved greatly nowadays by China to the general design level of Kitty Hawk for example even without any new lesson learned from Liaoning. Liangning main purpose is to train naval aviators, ground crew and CBG, not for improving design which can be done, in its most cost-efficient way, by deploying a spy or two.
Furthermore, if I were in charge, I would probably construct two 15,000 ton simple AC (one with the same deck the other with probably catapult, all with very basic equipments necessary) right now, to train take-off and landing only. In this way apart from tripling the numbers of naval pilots/ground crew and their experiences on take-off and landing, if theres any accident, god forbid, it would only happen on these decks instead of Liaoning. It therefore could greatly enhance the speed and quality of the real learning process of the training ship.