Do academic reviewers have a duty to critique authors who submit questionable research?
My feeling is that rather than explicitly telling the authors what to do in a context other than the present submission, it would be better to explain your objection to their present submission in a way which would make clear to any moderately receptive party that the advice would apply more generally.  E.g. something like


  In my opinion, the authors overestimate the significance of their work to a degree that becomes actively misleading: virtually no work in the field has the level of significance claimed by the authors here.   In general, work which is described in such hyperbolic terms can be hard to take seriously.  Those who do make major breakthroughs tend to describe the advances in more sober, factual ways.  


This is plenty heavy-handed.  Anyone who is listening will get the message not only to tone down the language in this submission but that such language is never appropriate.  Still, you aren't telling them what to do, but only giving your (expert) opinion on the consequences of their actions.