The hardware required to reverse enginer this chip is probably very expensive on its own, so they probably really need to put some money in line to attract the right people. It’s also a way of advertising their product, “Look, we promised $1.5M but even the best were not able to hack it”.
But if there is one thing I learnt, it’s that companies challenging hackers and researchers are always loosing, so I’m confident someone will find a flaw. It’s still Google tho, they know what they are doing when it comes to bounties.