"Therefore" ( Job 20:2) must refer to what Job had said. Job had previously asked why his friends answered him ( Job 16:3). Zophar replied that the spirit of his understanding made him answer ( Job 20:3 b). [source][source][source]
"This phrase means both that Zophar"s spirit is compelling him to respond to Job (c. Job 32:18) and that his words come from reasoned insight (cf. Fohrer)." [1][source]
Again he seems to be claiming innate, instinctive knowledge (cf. ch11). [source][source][source]