bbr does not infer bandwidth using delay; it just measures the amount of data asked between two points in time (ie. it directly measures the bandwidth). It also directly measures the rtt. The big insight in bbr is that these two values are all you need (and you convert them into a "pace" and a window size); unlike eg. Vegas, you don't slow down just because latency increases, you slow down when measured bandwidth decreases. This makes it far more resilient, especially to other competing TCP flows.