The quoted passage informs us that Thetis did not directly intervene against Poseidon, Hera and Athena, who, indeed, yes, should have been more powerful than she. A scholion on Apollonius Rhodius' Argonautica cites Ion of Chios as saying that it was from the sea that Thetis sent Aegaeon "up to protect Zeus". Your quote says that the hundred-handed monster merely had to sit next to Zeus in order to halt the attempt on the sky-god's freedom, the conspirators now too scared to try anything.
Ovid's Metamorphoses says that Aegaeon was a sea-god, and your Iliad quote implies a close connection between him and Thetis. On his Poetry in Translation website, A.S. Kline seems to build on Richmond Alexander Lattimore's translation (1951) of this portion of the Iliad, in which Lattimore translates the word Αἰγαίων᾽ as meaning the "Son of Aegaeus". Kline's translation renders the passage so that the monster whom men call Aegaeon is "mightier than his father Poseidon". Even though the word "Poseidon" actually does not appear in this line of the Greek text from which it is being translated, Aegaeus was incidentally a cult title of Poseidon, from the city where Iliad 13 says he lived in a palace under the sea.
In the Hesiod's Theogony, where this giant is called simply Briareus, or Obriareus, he antedates Poseidon and is this god's uncle, although by the end of the poem he has become Poseidon's son-in-law also. Briareus and his two triplet brothers are here repeatedly described as superlatively powerful: "irresistible was the stubborn strength that was in their great forms", and that they possess "unconquerable" and "overwhelming" strength. Apollodorus says that "They were unsurpassed in both size and power". With all this at their disposal it was easy for them to help Zeus win the War of the Titans. And as far as Homer is concerned, it takes merely the appearance of just one of these humongous entities to frighten off some of the powerful deities, such as Poseidon, Hera and Athena are.
As for further detail on the story of trying to bind Zeus, Timothy Gantz breaks it down as follows in his book Early Greek Myth: A Guide to Literary and Artistic Sources (1993:59):
This story survives only in the Iliad, and leaves much to be
explained (for example, what the conspirators might have hoped to
accomplish); if it is not a spontaneous creation of Homer, it could
refer to a time earlier in the Trojan War, when the gods in question
(these are the Achaian supporters) plotted to gain the advantage for
their own side. Ion of Chios seems to have told the story as well, or
at least mentioned the summoning of Aigaion [Aegaeon] to Olympos
[Olympus] by Thetis (741 PMG); we cannot say whether he had other
sources besides Homer. Though the Iliad scholia also reflect
puzzlement, the A group recounts Didymos' story that Hera, Poseidon,
Athena, and Apollo plotted against Zeus because of his high-handedness
and outspoken nature; when Aigaion saved him he hung up Hera in her
own chains and bound Poseidon and Apollo to serve Laomedon (ΣA
1.399). The b and T groups also know this last part, and add that some scholars actually substituted Apollo's name for Athena's in the text,
on the grounds that she would not threaten the father to whom she was
so close. But most of their commentary is devoted to an allegorical
explanation of the conspiracy, suggesting that they had few real myths
to fall back on. In the Iliad itself, we should note, Hera's
suspension in air stems from an entirely different matter, that of her
driving Herakles [Heracles] to Kos [Cos] in a storm.