No it isn't.
If $Author writes: "The foundations of the notion of fluid modernity (as outlined in the previous chapters) is fundamentally the simultaneity of dichotomies".
And I quote them as: "The foundations of the notion of fluid modernity [...] is fundamentally the simultaneity of dichotomies". I am not in any way changing their intended message. The author in that case reminded their reader about something they read in the previous chapter, something my readers cannot be reminded of, unless they also read the book recently. Therefore skipping that part with an ellipsis arguably would even be in the interest of $Author, because it isn't their quote that throws my readers off without $Author's fault.
This is just one example. Another very typical case would be if I'd write about $Person and quote someone who wrote "Jules Favre traveled to Königsberg in 1871 to negotiate peace with the Prussians, where he was met with ..."
Then if I'd write in the context of my text: "[He] traveled to Königsberg in 1871 to negotiate peace with the Prussians, where he was met with ..." wouldn't this again be totally okay if it improves readability in the context of my text?
The stuff I outline here is extremely common in theoretical, philosophical or historical texts. The existence of abusive misquoting doesn't mean one cannot use brackets in a responsible and good way (and you can also misquote without brackets).