William F. Porter isn't listed in the records of Hill's 7th Arkansas Cavalry. As you correctly pointed out, the records of a number of Confederate regiments were lost or destroyed during Price's retreat from Missouri.
Given W. F. Porter's age, as reflected in Mark's roster, he wouldn't have been eligible for military service until 1866, but he apparently managed to join the ranks in late 1864, just in time for Price's Missouri Campaign. The last surviving muster rolls from Hill's regiment included all men who had enlisted prior to March 1864. William obviously enlisted sometime after that, which puts him into that huge category of Arkansas soldiers who served but have no official record.
That's why postwar rosters, like the one Mark found, are so critically important. They may not be "official," but they often constitute the only evidence that a man served in the Confederate army.