At what concentration is sodium bicarbonate a sanitizing solution?
Sodium bicarbonate alone is not a sanitizer at any concentration, although it may be an effective cleaner, mostly due to its abrasive quality.  It may be used in certain sanitization regimes, in combination with other chemicals and agents.

Food Safety Site of Clemson University does not list it as a sanitizing agent.  According to 21 CFR 178.1010 - Sanitizing solutions (as queried through VLex), it is a part of two sanitizing regimes, but not the only component.

There are many non-credible, non-scientific sites that seem to claim sodium bicarbonate has anti-fungal properties, but I do not consider them worthy of consideration.

In terms of reputable sources, there is some evidence from NIH that sodium bicarbonate, at concentrations of 3%, can contribute to anti-fungal properties (at least for storing oranges), but it is part of an overall plan also including biological antagonists, not used alone.

I would not take this to mean that sodium bicarbonate is a sanitizing agent, which is a much stronger statement than simply reducing growth of fungus, which is only one possible type of pathogen.  I simply cannot find any evidence to support this stronger statement.