The gas flows over the tube past it at B. But at A the gas becomes stationary as the gas will move into the tube which already contains gas.
In applying Bernoulli’s theorem we should remember that p/ρ+1/2 v2+gz is constant along a streamline. In the present case, we are really applying Bernoulli's theorem somewhat indirectly. The streamline at A is not the streamline at B. Nevertheless the result is correct. To be convinced of this, we need only apply Bernoulli theorem to the streamline that goes through A by comparing the situation at A with that above B on the same level. In steady conditions, this agrees with the result derived because there cannot be a transverse pressure differential.