I think there are multiple possible reasons for the difference. As many people have said, they could just be passing savings on to the consumer in an uncharacteristic move. They could be trying to differentiate the lower end vs higher end 13” MBP, as the current M1 MBP is the 2 port model while the 4 port model is intel only (previously the chip difference between these two models was 8th gen vs 10th gen intel). They could also be trying to incentivize people to get Apple Silicone to make the transition quicker.

There is also the possibility that the M1 is slower than the 10th gen i5, although unless Apple was being intentionally misleading on stage this is not the case (have to wait for benchmarks to know for sure though).