It was my understanding that congress was going to vote on a no, not a yes of the deal. So, they are not voting to approve, but voting to deny which is why they needed only 34 votes. A filibuster would not help, because the president could still move forward.
Congress wants this deal, but they want to act like they dont which is why they went this route. The vast majority could vote no, and the president could veto their vote, and it would only require 34 votes to back up the veto and thus approve the deal.