Not sure about the graphene thing…
When manufacturers claim X minutes to 50% they’re just not taking the CV phase into account, in theory you could charge any modern lithium ion chemistry like NMC at up to 5C just fine, which translates roughly into 15 minutes to 70%. They could also lower the capacity while maintaining stupidly high charge rate in order to fulfill their claim, who cares about the long term capacity loss right?
We are just used to see very slow 0.2 or 0.3C charge rate because until just recently powerbanks were limited to 18-24W which was the max input for Quick charge technology and the upper limit for the MicroUSB port, but now that power delivery has become common for laptops we are seeing up to 3C charge rate for powerbanks.
I have a zendure super tank which is one of the first 100W capable powerbanks in the market, at first I was skeptical because it uses low drain LG INR-F1 cells but after extended testing I must say it works quite well, but at max output or input it gets really hot and the efficiency takes a hit.