Although the benefits of diverse forest systems are well known, many countries' restoration commitments are focused on establishing monoculture plantations

This right here. It's a shitty strategy, and pretty much lip service in terms of efficacy. 70% difference in efficiency aside, a monoculture doesn't promote the use of that land by diverse wildlife. A monoculture also puts the entire stand at risk for disease or some other thing wiping everything out. Just look at the mountain pine beetle for instance. While the areas decimate by pine beetle had 'monoculture' canopies, that had the right species in the understory and seedbank for those area to return eventually, after the die off.

The reason behind the 70% figure is likely that diversity promotes better access to some soil nutrients or provides better growing conditions/microsites and thus the biomass of the systems increase as a result..

Chen (2022) talks about how soil P is bolstered in diverse stands because you get a mix of deeper rooting plants pulling P from subsoil horizons to shallower depths, making it much more accessible to the majority of the forest spp.