So, to try and sum up that sequence of events:
*The receiver caught the ball and made three steps.
*The receiver then fumbled it before his knee hit the ground, but the referees blew the play dead because they thought it was an incompletion.
*A defender would 100% have been the one to recover the fumble, but they instead let the ref pick it up.
*The review shows it was a catch and fumble. Which means by the letter of the law it should have been Bears ball at the spot of the fumble (screwing the Eagles out of a turnover).
*The referee instead mumbles something and treats the play as an incomplete pass without actually saying as much.
I agree with Al Michaels on this one. "Are you sure they're not just making this up as they go along?"