Interestingly, you need to actually run commands for them to work.

config/waybar/modules.jsonc

···
 		"menu-file": "~/.config/waybar/power_menu.xml",
 		"menu-actions": {
 			"lock": "swaylock",
-			"suspend": "echo loginctl suspend",
-			"logout": "echo niri msg action quit",
-			"poweroff": "echo loginctl poweroff",
-			"reboot": "echo loginctl reboot"
+			"suspend": "loginctl suspend",
+			"logout": "niri msg action quit",
+			"poweroff": "loginctl poweroff",
+			"reboot": "loginctl reboot"
 		}
 	}
 }