We won't get to know the cause of the failure but at the end of the day, all cars are mechanical and therefore culpable to failure. Whether that be due to poor design, poor maintenance, bad luck or whatever, a engine failure can happen to any make, any model, any manufacturer.
This one failure does not mean that Lexus cars are unreliable, not unless it's a common route cause across many engine failures, which I don't think is the case.
Conversely, running an engine at max load, for any realistic amount of time (1 or 2 hours say, which is some going anyway) is still not pushing it to its limits. In engine development, engines are run for hundreds of hours at max load on rigs, so it should not in itself cause failure.