This answer makes no sense. Sound comes in waves with crests and throughs. In a controlled setting you can harmonize which effectively matches crests to crests and throughs to throughs. This happens in music with choirs, orchestra etc. In that case indeed sounds adds up.

However, with many random sources at many random frequencies you just get chaotic patterns where sound sometimes cancels out and sometimes adds up. No way that the overall result of this is that it adds up in dB. My first approximation would be that if one source emits 1dB and another also emits 1dB, these roughly add up to 1dB.

A very easy test of this is whether you now hear a very loud buzzing in your ear because at this very moment many people talk at the same time. I guess you know the answer.