You can read Christie's executive order here. He provides a legal justification for his pension move at the bottom of page 2:
WHEREAS, during the course of a fiscal year, the Governor may take
steps to limit State spending if it appears that revenues have fallen
below those originally anticipated in the Appropriations Act by
ordering the Director of the Division of Budget and Accounting
(“Director”) to freeze spending and place items into reserve pursuant
to N.J.S.A. 52:27B-26
The statute he cites says this:
In order to protect against and meet emergencies that may arise during each fiscal year, the commissioner shall have the power to set aside a reserve out of each appropriation, the exact amount of which shall be determined by him. Any time during the fiscal year that occasion may require this reserve or any portion of it may be returned to the appropriation to which it belongs, providing the commissioner finds such action necessary.
Whenever it appears to the satisfaction of the Governor that revenues have fallen seriously below those anticipated, the commissioner, on order of the Governor, shall have the power to revise the quarterly allotments.
As far as whether Christie's move is illegal, it may be. The New Jersey teacher's union doesn't think it's legal, and is planning to sue the Christie administration for violating the 2011 legislation you cited. So we may get a court ruling on this in short order.