Most aren’t shooting for an oligotrophic body of water, keeping nutrients down is a goal, but not to the point that the body of water would be considered oligotrophic. Especially in plant nutrients unless keeping a DKP, with a eco pond most are going for a balance.The beneficial bacteria bottles are enzymes that promote and target certain types of nitrifying bacteria.
After servicing hundreds of ponds, building them, and using or not using bacteria enzymes I can confidently say that it is absolutely a benefit and not snake oil.
Providing a steady diet to these bacterial colonies along with adequate aeration and circulation will keep your water column healthier than without.
Limnologists will argue this as well. If your goal is a oligotrophic body of water then adding in bacterial enzymes on a regular and consistent basis is 100% beneficial.
Whose to say it’s a benefit or not idk, I know it is not necessary to maintain a healthy pond if the design, ie nutrient uptake, circulation, proper filtration/aeration, proper stocking levels and understanding of the what’s and whys are taken care of from the beginning. So regardless of snake oil or not, it is a waste of money imo (th shelved bacteria in a bottle not the refrigerated stuff like fritz-zyme) as it is unnecessary.