The scan says size. It does not say that the electro magnetic shock wave packed the combined power of 50 supernova's.
That's true. However, we must remember what a supernova does -- it explodes and spreads as much of the solar matter possible over the largest distance possible. It doesn't control the radius of its blast like a bomb could.
Now, for a bomb to explode that 50 times over, it would at least have to have 50 times the force behind it. It could be more.
To complicate things further, it could also be less depending on how much force was in each inch squared. Of course, it could be more in this situation as well.
Of course, not everything is properly answered in the panel, but we all could agree its a very, very good feat, and I'm not necessarily asserting Superman tanked the power of 50 supernovas exactly, but I think its fair to assume he can take, like, 10 😄
If you look at the last panel in the scan, it mentions him escaping.
Nope, it shows him running away from the radiation, which, by nature, isn't a contributory force to an explosion.