I don’t think the calculator is including the sonic boom from the passage, just the explosion.
However, an article in Nature states the blast damage was from the explosion, not the sonic boom:
http://www.nature.com/news/russian-meteor-largest-in-a-century-1.12438
On further thought, I suspect that the overpressure was much higher than the calculator because the (simple) calculator may simply have the air blast effects be spherical, when in reality they were focused along the path.
The paper explaining the calculator is quite technical and can be found here:
http://impact.ese.ic.ac.uk/ImpactEffects/effects.pdf
The calculator notes, also state that the minimum velocity at impact is 11 km/sec (I’m buying it), but that the maximum velocity for a solar system object is 72 km/sec. Classically, the maximum velocity on impact with the earth for a solar system body is 44 km/sec = sqrt( 2*(mu_sun/R_earth_orbit + mu_earth/R_top_of_atmosphere))
The maximum orbital energy for an object to be graviationally bound to another body is 0. More than that and it has escape velocity. When the energy is 0, the velocity is:
v = sqrt( 2*mu/R)
For earth mu = 398600.4418 km^3/sec^2, R ~ 6491 km from center, = 11 km/sec
For the sun mu = 132712440018 km^3/sec^2 , R ~ 150,000,000 km from center, = 42.07 km/sec
The energy adds, regardless of path, so the resultant velocity is root of the sum of the squares, or about 44 km/sec
Spherical dissipation goes as 1/R^3, line source as 1/R^2, so you may be on to something.