You are correct - lots of folks call bacteria in a bottle "snake oil". My opinion is it's both that and also can be helpful. The idea that you need to continuously add bacteria to an established pond seems snake oily, but to kick start a new pond, or to re-establish a bacteria base after some kind of pond event (maybe a full clean out was required, or you had to treat the pond for parasites, etc) I think it's not a bad idea. The concept of seeding a new tank with a used filter has existed in the aquarium world forever. On the grand scale the only thing that's hurt if it doesn't work is your pocketbook.
I mean, my gut is loaded with bacteria (as is everyone else's on the planet) but I still take a probiotic every day and eat lots of fermented and cultured foods to maintain my gut health. Snake oil? Maybe. But when you learn that bacteria outnumbers human cells in the human body by 10 to 1, you realize A. how important they are and B. that you want to make sure you are supporting the good stuff so the bad stuff can't take over. I've read so many things about the gut biome and how "ancient" cultures (living in a modern world but without all the modern foods, medicines, chemicals, etc) have hundreds of varieties of gut bacteria. The average "modern" human can have as few as THREE. You can see how easy it would be for one bad actor to overcome the system when there are so few "good guys" to fight it off.
Just my own thought process on the "should I or shouldn't I" of adding bacteria in the pond. It would be interesting to be able to test your pond for bacteria, wouldn't it?