Yes. But this is from 1908. His opportunism was revealed in 1914. Until that point, he was seen as the leader of the Marxist movement. For instance, when Lenin learned in 1914 what Kautsky'd done he dismissed it as misinformation. There was a reason people liked Kautsky.

Further, I think that pinning all of the opportunism on Kautsky fails to explain the Second International's opportunism and, most importantly for us today, its origins.

I guess you should read the preface and consider whether it fails to put forth a properly internationalist perspective and whether it ultimately dismisses a revolutionary class struggle in favor of class collaboration.

Beyond that, Herman Gorter, at the very least, can't be said to be an opportunist.