I think that Apple "just" [1] picked a very happy place in the parameter space: lower clock means that main memory (which is already lower latency than usual) is "nearer" to the cpu plus a larger L1 with a shorter latency (3 cycles). These (plus probably a larger amount of sustainable outstanding misses) imply that the cpu stalls less which means that they can sustain a larger ILP [2]. A better branch predictor (which is rumored to be best in class, although I haven't seen any in depth analysis) means that they can usefully have a larger reorder buffer and extract even more ILP and memory level parallelism. Finally the large width can take advantage of the extracted parallelism.

The weaker memory model should help extracting additional memory level parallelism, but I doubt that ordering has large impact. M1 is quite good even when running in TSO mode.

[1] of course they need to be commended for this, they went against the usual wisdom, probably because their CPU was designed to be low power first then high performance, not the other way around.

[2] typically the average ILP for normal applications is 1.5 instructions per cycle, but the variance is high, so if you reduce the amount of cycles the cpu is stalled, the ILP can go up fast.