omissibility - commas/ omitting "which is"

In the passage below, shouldn't the writer have included which is before the word championed, since it is in a non-defining clause?

In seeking to describe the origins of theater, one must rely primarily on speculation, since there is little concrete evidence on which to draw. The most widely accepted theory, championed by anthropologists in the late nineteenth and early twentieth centuries, envisions theater as emerging out of myth and ritual. The process perceived by these anthropologists may be summarized briefly.