What unique instrumentation change did Beethoven's Symphony No. 9 use that, according to the website, was a "radical definition of the form" of a symphony?

I would say that the answer is the introduction of a chorus into the form of symphony, which had previously been considered a purely instrumental genre. Beethoven was the first composer who dared to do this and was followed by Mahler, Bruckner, Shostakovich and other notable composers.