"High in reach" is a non-idiomatic way of saying that it reaches many people. This uses the word reach in the following sense:
2 The extent or range of something's application, effect, or influence.
‘he told a story to illustrate the reach of his fame’
Consider the fuller quote of the link you provided (emphasis, mine):
When all is said, the promise of the history of diffusion scholarship and diffusion practice is a promise of efficiency in intervention: Communicating an innovation to a special small subset of potential adopters so that they, in turn, will influence the vast majority of other potential adopters to attend to, consider, adopt, implement, and maintain the use of worthy innovations. Our interventions must be high in reach but low in cost in order to most persuasively demonstrate worth in intervention.
It's talking about accessing a small group that would reach the larger (target) audience.