28,000 atmospheres isn't all that great. There are planets out there with tens of millions of atmospheres pressure. And yes, that's about the same pressure as at Jupiter's core. They're just extreme outliers of the Stellar Forge planet generator, and demonstrate that the Stellar Forge doesn't have a "planetary reclassification" algorithm - because a planet generated with several million atmospheres ought to be reclassified as a gas giant. Or in this case, since the atmosphere is mostly water, probably a Water Giant.
The high temperature on the surface of this planet is a product of the super-thick, entirely-water atmosphere. Water is an excellent greenhouse gas and planets with mostly-water atmospheres are usually extremely hot.