Tensile strength is part of the issue but is really just how the chains are rated. You could tow or pick up your bike with a length of chain from the average mountain bike. Look at the mechanism on a fork truck or bike lift, it really is bicycle chain. What motorcycles have an issue with is impact loading (when you go from full throttle in first to the brakes), fatigue (repeated loading) and vibration (smaller parts have a natural frequency nearer where a bike revs). The F650 at 50 HP is probably as bad on a chain as say an 80 HP F800 because the single cylinder motor vibrates due to the single big pot, produces power in a lumpy way (one big pop in 720 degrees of crank rotation) and having less power probably has it applied in bigger doses. You won't snap the plates on the chain or collapse the pins as you would in a tensile test (big hydraulic puller snaps it), you will wear the pins or holes until the pitch isn't constant enough to work by repeated small loadings. Grease held in there doesn't make the chain stronger in tension, it prevents the wear by acting to "float" the pin in the hole.

The O-ring and X-ring chains have higher rated tensile stength because the materials are designed to work when lubed, as are the tolerances. I'd bet the pins are tighter in the holes so the compressed seals work. There is also IMHO an element of higher tensile strength looking better that a story about grease on sales literature, so design fudge factors that go with the seal set up are expanded upon to give a readily understood reason why one costs more. I'd bet if you tested any 520 chain it'd take over 10 tons. DID won't tell you than in case your application hits 9.5, does a sudden jerk and breaks one.

From memory (The last time I was a mechanical engineer about 15 years ago), the size of chain used on a 50 HP bike, in an oil bath would be specced for a 250HP electric motor. The tensile strength is still easily enough, but the wear is almost zero due to lube and smooth power delivery.

The chain case holds the grease, so you don't need the spray on lube, o-rings and so on. If the chain case leaks the grease will wash out leaving you with an unlubed chain, or sand will get it leaving you with contained grinding paste and probably a worse case than an unlubed open chain. Fitting a dry O-ring chain in a case is better until the case fills with sand, then it's worse. To me if you cover the chain it wants to be done fully and packed in grease.

Andy