The Universal Century timeline began, in our experience as the audience, with the One Year War, depicted in the original Gundam series. In a battle between the Earth Federation and Principality of Zeon, the faction that the show designated to us as the enemy was Zeon, and were presented like their super robot enemy contemporaries: comical and cartoonish in nature. All future shows placed in the middle of this war seemed to work as apologies for this initial depiction. War in the Pocket, Stardust Memories (it's still pre-Gundam Z), 08th MS Team, and Gundam IGLOO all served to humanize the soldiers of Zeon in one way or another. The lore's predestination of a Zeonic defeat paints these soldiers doomed by their commanders and by fate and victims of past Federation abuses fighting the good fight while crying "Seig Zeon" with a sense of national pride.
This depiction is what made the battle cry of the final remnants of Zeon forces on the earth's surface so painful. These were soldiers that were members of the initial invasion forces who was left behind and forgotten and never accepted the peace that was signed oh so far away. They missed out on Operation Stardust. They missed out on both Neo Zeon uprisings under the lead of Haman Karn in Gundam ZZ and Char Aznable in Char's Counter Attack. This attack on Torrington was their last chance for not glory, but vindication and vengence.
The attack was never going to succeed. Their ancient mobile suits were never going to stand up against London Bell's crack troops. Their numbers were never going to hold out against a Federation counter attack. But it did do what they wanted: get one last punch in. The chants of Seig Zeon in this case was no longer a chant of national pride. It was merely a tired symbolic title to serve as cover for a personal vendetta. Loni's last desperate chant of Seig Zeon was but a ghost compared to the chant that Ghiren inspired so many years ago at the dawn of this long lived conflict, and I can't help but pity them despite being a Feddie at heart.