When you read this verse remember that Solomon had hundreds of wives and concubines. He still wrote this verse. Although the Bible does not forbid having more than one wife the New Testament does seem to state a preference.
(Tit 1:6 KJV) If any be blameless, the husband of one wife, having faithful children not accused of riot or unruly.This admonition is given three times in association with the offices of bishop, deacons and elders. Solomon might have done better to read what Paul had to say.
So? Truth is truth even if the speaker isn’t living up to it.