Dr. Zakir Naik is a controversial Islamic preacher, and his views on the authenticity of hadiths may not be accepted by all Islamic scholars.
As for the question of whether all the hadiths in the Sihah Sitta are authentic, it is important to note that the Sihah Sitta (which includes Sahih al-Bukhari, Sahih Muslim, Sunan Abu Dawud, Jami at-Tirmidhi, Sunan an-Nasa'i, and Sunan ibn Majah) are considered the most authentic and reliable collections of hadiths in Sunni Islam. However, not all hadiths included in these collections are considered authentic by all scholars.
Many scholars have studied and analyzed the hadiths in the Sihah Sitta and have categorized them according to their level of authenticity. For example, some hadiths in these collections are considered sahih (authentic) or hasan (good), while others are considered weak or fabricated.
Therefore, it is important to consult with qualified Islamic scholars and experts in hadith sciences to understand the authenticity of a specific hadith or collection of hadiths. It is not appropriate to make sweeping statements about the authenticity of all hadiths in a particular collection without considering the opinions and analysis of the experts in the field.