Colin Percival has credentials and experience in the cryptoworld. When he makes "new" crypto like scrypt he publishes it and it's been thoroughly reviewed. It also has distinct advantages over previous technologies, it's not just new for the sake of being new.

He also, AFAICR, did not appear over confident, he was clear in delineating what his application does and does not protect against, and what his goals are (that is one reason why he deserves these crypto credentials).

He published his entire source code.

And when he launched a contest, it was in the form of a bug bounty, he accepted any kind of bugs (up to and including spelling errors in his code comments :) ).