They mean that:
(a) ACW saw widespread use of rifles, to the almost complete exclusion of smoothbores by 1863-1864, expanding the range of combat and ensuring fire, rather than melee (mainstay of TW), decided almost all combats.
(b) Cavalry (specifically, heavy cavalry, lancers, and the like) saw minimal use on the battlefield, instead being relegated to heavy infantry status, and removing one of the arms of the combined arms triad in use to that date
(c) That, give the two points above, Civil War combat is somehow less "exciting" than Horse and Musket period engagements. Personally, I disagree, as I find fire an maneuver by infantry heavy units just as exciting and dislike melee combat, but that is me.
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