Payoff?
FRAND royalties would hardly begin to compensate them for $600 million in development costs. But that's not the point, or the reason companies spend money like that to develop technology that must be offered on FRAND terms to be allowed into a standard. Huawei would earn a lot of international prestige for themselves and for China, and the value of that is such they'd probably be willing to license it royalty free. They'd also be experts in this technology, and have a jump on everyone else (especially rival Qualcomm) in developing chipsets that implement this.
Assuming it is as good as they say it is, and someone else doesn't come up with something better.