Lithium Ion batteries are notorious for overheating when charged or discharged too quickly. (especially cheap, poorly made ones).
But this shouldn't be an issue when charging with the factory charger which is (should be) properly matched for the right charging rate. So, either the maker knows that their charge are crappy and tend to overheat the batteries, or it's simply a boiler plate warning on par with "place in 350° oven for 1 hour........WARNING may be hot".
In your shoes, I'd charge it outdoors just in case, but not worry about the rest.
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