It is a probability that the Enterprise-A isn't the Yorktown that was mention on screen. But most likely the Yorktown that Captain April had commanded before he had commanded the Enterprise.
I'm conjecturing that the Yorktown that Captain April had commanded was a Constitution class starship that was decommission and put in moatball just before the Enterprise was commission.
I'm basing this on what Captain James T. Kirk had told Captain John Christopher; Saying that there was 12 others like the Enterprise in the Fleet.
The 12 other or:
The USS. Endeavour wouldn't had been commission at that time. But, because we had seen her registry number on the display wall at Starbase 11, that she was being built at that starbase. Then there is the USS. Eagle. Because she having a lower registry number? She was in moatball at that time. But later was taking out of moatball, refitted, recommission and put back on acting service in the fleet.