This is unlikely to be true unless a charger with a higher amperage than normal is used.

Mine is fine, though I do agree it has battery life issue like reply below.

Even sometimes I charge my mi band 5 while it's still wet and still fine. Maybe he's using bad charger?

OK
I am member of mi 3,4,5 group. I have seen this particular picture of mi 5 with description of exploding whilst charging. Someone replied, explained that mi 5 was hit with hammer.
Personally I believe this mi 5 owner did suffer the problem as described as she admitted to throwing item as well. If my xiaomi mi band 3 exploded I'd do the same. Plus owner reported to many online places, to make people aware. Mi 5 does have a magnetic charge, and I have seen magnetic usb phone charge cables. Maybe research needs to be on the agenda for mag-charge hardware.
Some usb items I have heard by irl by word of mouth do explode, like power banks, phones etc. I hope owner gets money back, plus an apology and compensation for being frightened and damage to affected . I advise owner to purchase mi 3,or 4,as it has a usb cradle charge as mine hasn't blown up YET! Sorry if I seem harsh.  ;)

I think the more logical question is whether the watch is hit or dropped. There are many documented ignitions of devices with lithium-ion or lithium-polymer batteries, when charged. Specifying that they had been hit before.

This is really interesting. I'm currently wearing mine and am on day 8 since last full charge. I don't use a power adapter though, my power bar has 2 USB slots that I just plug the cable into.

Definitely gonna keep an eye on mine though just in case. I wonder if the one that exploded was exposed to water in any capacity beforehand?

I find this interesting and worrying as mine isn't holding charge very well. It's down to about 5 days. It would be interesting to know if he was having any battery issues before.