That is true it was. Most of Holmes attention at the time seems to have been focused on Hindman and the problems he was having after the Battle of P. Grove. Bill Shea's recent book outlines some of these problems of Hindman's retreat.
Arkansas Post seems to have been a Ill concieved place to built such a fortification because it was exposed. The only reason that I can see for building there is the deep water channel from there to the Mississippi River. A secondary reason was the need to protect the Corn crop of the lower Arkansas River valley and to establish a communication link with the east. I believe that had Arkansas Post survived until the soring flodds of 1863 that Holmes would have moved the CSS ponchtrain to Arkansas Post as a threat to Grants operations at Vicksburg. Commander T.O. Selfridge was very concerned about the Ponchartrain even with it being bottled up at Little Rock. But that is all speculation.
As for Walker and Parson's certainly their presence in side Arkansas Post would not have mattered as far as numbers go. But one of the major factors of any Battle is the thing called "the Fog of War". The fact that your enemy does not known what he does not know. The fact that a confederate relief force was gathering may have actual caused the Federals to abandon their attacks had the Post Garrison held out just a little longer.