No, it can charge while playing when using the official adapter. It's just that people were using low-rated battery packs. USB C can support up to 100W, the Switch charger is 45W, so if you're using a pack that doesn't output more than 10-20% of that, obviously you'll have slow charging and probably not enough to sustain playing and charging simultaneously. I'm not worried, I'm almost never away from power for more than 3 hours anyhow, though I will be finding a high capacity battery that can output a good enough charge rate.
Yah, you're right. Quoting from the review: "we’ve yet to find a battery / cable that can charge the console faster than it drains." So it's possible and likely that someone will make a capable pack at some point.
On the positive side, plugging a powerbank into the Switch causes the console to recharge faster than it depletes its own battery even in a stress test scenario with brightness and volume maxed. [1]
Looks like it won't take much. A lot of USB-C power banks on Amazon max out at 3A/5V; at 15W, that's just about the top power consumption measured on the Switch (16W), so it ought to be enough to at least keep the battery constant as power use fluctuates during regular gameplay.
I expect we'll see 5A+ power packs in the future which will handle it easily.
But the Switch downclocks the GPU greatly in portable mode, so it consumes less power. If it does that, and charges but a bit slower, you won't need a 45W battery pack.
