I didn’t want to plug the charging cable in to my MacBook based on these claims so instead I used a mains powered charger.
I left the speaker on charge for about 40 minutes and documented the results in the video below. After this time the speaker and the cable hadn’t even warmed up. The temperature remained unchanged for another 45 minutes after the video was shot while still plugged in to the USB.
Of course I’m not denying that this did happen to one of the devices but it seems that it happened in isolation as it’s the only report like it I’ve seen and I cannot replicate the problem.