Sure, and there's some degree of truth there. At the time she said this the Sin'dorei had a recent experience with siege warfare, specifically the Scourge; however, the usefulness of that experience is
severely limited. The Scourge were not known for their tactics, typically winning wars through attrition. Although, later, we would find that the Kaldorei sentinels were more than capable of laying siege to a capital, as they were the ones who would go on to breach the defenses of Orgrimmar during its siege by the joint Alliance and Horde forces. That said, even if we accepted that the Kaldorei would not be able to breach their gates, there's a question as to whether that even matters. If the Sin'dorei are pushed back and confined to Silvermoon and cannot leave for fear of being hunted by sentinels and ancients, is that a victory? The longer the Kaldorei are in Quel'thalas, the more treants and ancients they can call upon, leading the a viscous cycle of the longer they are able to hold their ground, the harder it becomes for the Sin'dorei to fight back (and the harder it becomes to fight back, the longer the Kaldorei will be able to hold their positions).
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The sentinels in Quel'thalas were sent as spies, not to act as a major military presence. Undermine and sabotage the Sin'dorei? You bet. But not to act as an opposing force. Unless you're planning to argue something equally silly, like that the Draenei would able to single-handedly defeat the entirety of Kael'thas' forces because they defeated the meager force that followed them to Azeroth, there's nothing to discuss here. A single outpost of Kaldorei is not an apt comparison.
Neji's Point is not some major, well supplied stronghold. It's an outpost that's cut off from virtually every other Alliance outpost, caught between Sun Rock Retreat and Shadowprey Village (both Horde outposts). Moreover, this was not some amazing feat of Sin'dorei magic. The quest
Firestarter, and the quest chain surrounding it, is quite explicit with what you're doing. You steal magical relics from a temple of Elune, charge the relics by destroying the spirits of ancient Kaldorei and forcing their power into it, and then unleashing the power stored onto Neji's Point.
Which is great, but in A Good War Saurfang states that the casualties of the joint Horde forces were higher than that of the Kaldorei, who were not at full strength and did not have the full support of the Alliance in Darkshore (due to logistics, not for lack of trying).
I'm cutting out almost everything because a
link to the questline is more than sufficient, there's no need to copy/paste paragraphs of text.
The Kaldorei in Azshara during Cataclysm are almost exclusively novices. This was not some great, longstanding Highborne camp which contained many ancient Kaldorei. This was a single Lorekeeper and her Kaldorei apprentices which, given the canonical timeline, would have been learning magic for less than a year. The only thing that's impressive about what was done was the decision of Lorekeeper Amberwind to undertake an expedition with a group of undertrained apprentices into Horde-occupied territory.