I doubt that 'wright-brothers.org' is a reliable impartial source on that. The idea of heavier than air flight had been around, it wasn't conceived by them. Lilienthal was an earlier well-documented pioneer, there are pictures of his gliders in mid-flight. Just to name one. So without doing further research, the idea that actual scientists at the time thought it was impossible seems highly unlikely to me. As early as the 18th century Europeans experimented with fixed wing "flying machines". The Wright Brothers probably weren't even the ones to first archive powered flight, there are multiple contenders.

It's a bit like with Elon Musk (re-)inventing the electric car. Those popular names that went down in history are usually not the original inventors. These people were first of all successful entrepreneurs that understood business and ultimately won the patent war. But in the early 20th century, the idea of flight was already firmly established, why would any "respected scientist" have doubted what could already be observed in action around the Western world?