The threading does reduce strength of the barrel (non fluted or uniform thickness) at the muzzle but only for a very short distance and more importantly the reduction is uniform, that is the thickness is uniform. If the fluted portion of the barrel was threaded the high and low portions may not be uniform so that the expansion is uniform. When the solid portion of the barrel near the muzzle is removed the barrel is weaker for a longer distance.
As others have noted there are numerous factors that can affect the degree to which the barrel is affected.