I am not aware of that research in particular, but we do know that even a couple of inches of mulch will dramatically decrease evaporation from the soil surface, leading to higher levels of soil moisture. That is one of the main reasons we recommend mulch. However, the mulch is unlikely to be the problem. There is an underlying drainage issue, which is unsurprising given the position of the house relative to the wetland area. 

Was moisture in the basement a problem prior to the major renovation? If so, were changes made to the grade around the house? Is it only a problem after particularly heavy rains, or is moisture a consistent issue? How old is this house? It is likely any foundation drains, if installed, have broken down or filled with tree roots. 

I suppose by the moat of gravel, the contractor is envisioning something like a "poor man's" French drain (see publication, below). It may have a place, but it is important to figure out the cause of the problem. If it is hydrostatic pressure caused by a seasonally high water table or saturated soil after heavy rains, that moat will do little. 

Making sure the grading is correct is the low hanging fruit. As for foundations drains, the city may have some information on how the house was constructed, and the Drain Commission may have suggestions on improving drainage around the home.

Some additional information is here:

https://extension.wsu.edu/clark/naturalresources/smallacreageprogram/improving-drainage/

https://extension.illinois.edu/blogs/good-growing/2020-02-12-landscape-drainage-homeowners