I looked up the quote OP is referencing, and could only find a wiki entry stating that the quote was from "a Union colonel," which is cited as from Harry Pfanz's book "Gettysburg - The Second Day" which I have not read. I'd defer to anyone who has read this book to provide more context on this quote to see if there is more information on the colonel who said this, and whether this observation was based on a person who had been in battle very little, or one who had fought in several. Who this colonel was, where he was on the field when he made this remark, and how prone he was to exaggeration would be helpful.

But let's just say that this unnamed colonel had been in several scrapes, and did truly believe that Barksdale's actions were some of the most heroic of all time. Why he might have thought this probably boiled down to the fact that Barksdale led his men into the teeth of a Union line on horseback (he may as well have put a bullseye on himself), and that they were by all accounts very tenacious in their attack.

Barksdale and his men hit the Union lines in a salient that had been created by Union Gen. Sickles extending his men out in front of the rest of the army. This meant that Barksdale's men hit an isolated Union position, drove it from its position (as one might expect since they were all alone), then plunged into the heart of the Union lines at an exposed point that was collapsing in on itself. This would have been a dramatic event to witness, and in a day where most Confederate forces merely got up to the Union lines, but did not drive them back all that much, this would have stood out as an exceptionally successful attack. Of course, the Union lines readjusted and were able to throw this attack back, like it did with all other Confederate attacks that day, but in the moment, it probably seemed VERY dramatic and almost poetic. I say "almost" because the carnage at this point in the field was horrendous, and the Union colonel who made this remark probably made it from a distance, watching the lines sway back and forth (Barksdale mounted at the front of his men like some kind of knight...before he was shot to pieces). Again, having not read Pfanz's book, I can only guess, but based on the fighting that day, and the disposition and placement of Barksdale's men, that is about the best I can figure.

[Sources: Bruce Catton: 'Glory Road' & 'Never Call Retreat'; Shelby Foote, 'The Civil War, vol. 2'; James McPherson, 'Battle Cry of Freedom']