Why would it(NO3 at say 2-5ppm vs 10-20ppm) depress CaCO3 deposits?

I want a really good answer that's definitive before I buy that.
Why?

The macro algae have the same process..they too deposit CaCO3 into their structure.

That arguement(not directed towards you personally BTW!) about corals is nonsensical, basically a non answer.

By this arguement, we should also see issues with macro algae and more intense and faster with higher NO3 levels.
But we do not...........

I think they have other issues they have not resolved and have not dosed NO3 via KNO3 or an inorganic source.

Equating inorganic KNO3 to fish waste loading is a bad assumption, very bad.

The bubbles in the DBS: likely N2 gas, you just gave the bacteria a lot of NO3 which they like.

So some of the NO3 is being transformed rather than assimilated into plant/macro tissue.

Regards,
Tom Barr