The Guardian doesn't give a citation so there is no way to evaluate the claim.
I don't think Sunnstein's views on speech are in general inconsistent with what you typically find on say HN: http://mtprof.msun.edu/fall1994/TrRev.html ("Sunstein's focus is on the relationship between the media--newspapers, television, radio--and political culture. For him, the media, increasingly owned and controlled by a dwindling number of privately owned conglomerates, are in the propaganda and bread-and-circuses business. They make sure that the American public is not exposed to radically new viewpoints and that it is distracted from serious issues by an endless array of tabloid topics, from Nancy Kerrigan's knee to John Bobbitt's penis.") See: http://mtprof.msun.edu/fall1994/TrRev.html.
He favors restrictions on e.g. hate speech and political speech by corporations, which is not inconsistent with what many (most?) liberals believe. But he's also made it clear he thinks such restrictions should generally be based on harm, not ideas. http://www.gvpt.umd.edu/lpbr/subpages/reviews/sunstei2.htm ("A subject matter restriction on unprotected speech should probably be upheld if the legislature can plausibly argue that it is counteracting harms rather than ideas."). I don't agree with him on any of these points, but his views are not outside the mainstream, and are consistent with those of say European countries that have outright bans on hate speech.
I don't think Sunnstein's views on speech are in general inconsistent with what you typically find on say HN: http://mtprof.msun.edu/fall1994/TrRev.html ("Sunstein's focus is on the relationship between the media--newspapers, television, radio--and political culture. For him, the media, increasingly owned and controlled by a dwindling number of privately owned conglomerates, are in the propaganda and bread-and-circuses business. They make sure that the American public is not exposed to radically new viewpoints and that it is distracted from serious issues by an endless array of tabloid topics, from Nancy Kerrigan's knee to John Bobbitt's penis.") See: http://mtprof.msun.edu/fall1994/TrRev.html.
He favors restrictions on e.g. hate speech and political speech by corporations, which is not inconsistent with what many (most?) liberals believe. But he's also made it clear he thinks such restrictions should generally be based on harm, not ideas. http://www.gvpt.umd.edu/lpbr/subpages/reviews/sunstei2.htm ("A subject matter restriction on unprotected speech should probably be upheld if the legislature can plausibly argue that it is counteracting harms rather than ideas."). I don't agree with him on any of these points, but his views are not outside the mainstream, and are consistent with those of say European countries that have outright bans on hate speech.