There is nothing inherently implausible about breaking chains on an MTB. In early days of mountain biking chains broke with great regularity. Anyone with any sense always carried a chain tool and they got used. Most of the chain breakage would be from jumping hard for a climb or obstacle while shifting. MTB derailleurs in current production are enormously better at chain management than what we used thirty years ago. Low end bikes from Walmart will have derailleurs not particularly different from older derailleurs. Or rather derailleurs with older design and no quality.
Better shifting technique helps a lot. Better derailleurs matter just as much. Almost any new derailleur on above entry level bikes will not have the problem in OP. The Walmart bike will have other problems that overload chains. The alignment is unlikely to be much good. The chain is very likely to be substandard. Tooth shape will be basic.
Chains break at around 1500 pounds of pull. When pulling straight. When the chain is out of line, snarled, snatching against cogs or rings, they survive a whole lot less. Persons of normal strength can do it. Good bikes made the past twenty years and better than abysmal shifting technique it is real unlikely. But it still happens once in a while.