What he said is just his opinion, an opinion from one person who may be successful and influential but still one person who may commit mistakes and do wrong judgements. From the title of the article "Warren Buffett says bitcoin is 'rat poison'", well rat poison could only kill rats but it could also harm humans if ingested accidentally. How Bitcoin can be viewed depends from each person, for me Bitcoin is a new technology and an evolution of the money system but of course everyone will accept it as some people aren't willing to accept changes and just cling to what they are used to.