"public offer" includes initial public offer or further public offer of securities to the public by a company, or an offer for sale of securities to the public by an existing shareholder, through issue of a prospectus. When an existing shareholder sells his shares, he will do so on a stock exchange. How does a prospectus play any role in selling of securities on a secondary market?
You will study this in another video - 'Sec. 28 Offer of sale of shares'. When a significant shareholder wants to sell shares such as promoter of company, doing so directly on stock exchange won't be possible or recommended because of fluctuations, in that case, company does it.