Enzymes are usually specific to a greater or lesser extent for their substrates. In this case we are looking at citrate synthase which catalyses a condensation reaction in which the acetyl group from acetyl-CoA is added to an oxaloacetate molecule to form citrate. This is the entry point for carbon into the TCA cycle for energy production. The condensation reaction follows an ordered pathway: oxalocatetate binds first and this results in a conformational change which increases the enzyme's affinity for acetyl-CoA by a factor of 20.
Citrate synthase is highly specific for acetyl-CoA as a substrate. The cited reference discusses several aspects of this specificity. The enzyme has a well-defined binding site for CoA, with contacts from both the small and large subunits. This includes the recognition of the adenylyl portion of the molecule.
A relevant quotation from the review:
"Citrate synthase is highly specific towards its substrates. The condensation reaction occurs only with the physiological substrates oxaloacetate and acetyl-CoA."
So, the answer is that citrate synthase cannot use acetyl-phosphate as a substrate because of its specificity for acetyl-CoA.
Wiegand,G and Remington, SJ (1986) Citrate Synthase: Structure, Control, and Mechanism. Ann. Rev. Biophys. 15: 97–117