On screen feats it was shown to be stronger than Tsar Bomba- I’m pretty sure anyways.
The yield of the Atlantis nuke was
GIGANTIC, blowing apart the enormous mountain-temple with dozens-hundreds of meters thick walls (themselves hundreds of meters away from ground zero) that Godzilla was in, then the shockwave was still able to make a cavitation bubble dwarfing said temple in the surrounding water, propagate through kilometers of underwater tunnels to destroy the sunken city and then
still have enough energy left over to launch a large submarine up to the ocean surface and most of the way out of the water.
Ignoring the fact that even what we see reaching the open ocean at the end of the sequence actually appears to still be a very brightly glowing cavitation bubble rather than just a shockwave (which from what I understand has pretty crazy implications of firepower that would definitely put the yield in the gigatons), I’m pretty sure just the way that it blows up Godzilla’s mountain-temple and the surrounding city and how it still has enough force to launch the sub like that at that distance is still enough to put it well past any real nuke.