I doubt he was paid, as he maintains no ongoing relationship with Uber and it’s in the interest of his wealthy employer (Princeton) to publish interesting results.
Alan Krueger is also among the most respected labor economists in the world. With David Card in 1994, he published the first major research suggesting that minimum wage increases don’t always reduce employment. He then went on to be President Obama’s chief economist, among other roles. He wouldn’t risk his credibility on Uber.