Most states were not formed out of independence movements.

True. And no one claims they were.

Rather, the point is that independence movements that seek self-governance and autonomy for a self-identified group of people are generally not dismissed as "a mythological claim about an essential identity." Only Zionism, for the most part. There's no "Antitibetanism," deriding the effort to liberate Tibet as a purely political project seeking to create a horrific ethnostate, for example. Progressives aren't denigrating Palestinian self-identity as a mythological claim that's part of a political project to establish their own ethnostate. There's no "Antijapanism" or "Antibangladesh" attacks on those countries' legitimacy or existence.

Just the Jewish one.

Which is why when folks express support the specific concept of "Antizionism," as opposed to arguments against ethnostates generally, it is perceived by some Jews as antisemitic. Because it is singling out a specific group as not being allowed to advocate for its own autonomy.