His instructions were to move if attacked.
See this:
D. C. Buell, Assistant Adjutant-General at Fort Moultrie, South Carolina, December 11, 1861: "...An attack on or an attempt to take possession of any one of them (the forts) will be regarded as an act of hostility, and you may then put your command into either of them which you may deem most proper...."
There was no attack.
Anderson had no instructions to move his force on December 26.
Buell's comments were to move without provocation, which, as can be seen, he did not have the authority to make an order of same.
So, despite your assertions, there was no order for Anderson to move on the 26th.