I can not understand the part of the paragraph because I think they are contradiction or irony. Could you explain the process of it? Thank you!
I think you're misinterpreting the passage.
Gas giants form via a process know as core accretion. First, a massive core forms from solid material, initially with a small amount of gas. The envelope then grows, producing an object we would recognize as a gas giant - a large gaseous atmosphere, surrounding a comparatively small "rocky" core.
The problem with Hot Jupiters is that gas giants, according to current theories of planet formation, cannot form extremely close to a star. Inside a boundary called the frost line (which changes over time), there is not enough solid material for such a massive core to form and accrete enough gas to form a gas giant before the protoplanetary disk dissipates. Therefore, any gas giants that exist inside the snow line should have formed outside it, and then traveled inward via a process known as migration, which can happen through several different mechanisms.
The phrase "spiraled inwards" doesn't mean that the planets will spiral into the star, merely that their semi-major axes will shrink and their orbits will move closer and closer to their parent star, possibly stopping at some finite distance.
Like David Hammen, I'm a bit confused as to whether you think this happened in the Solar System, or just other exoplanetary systems. As far as I know, this sort of formation and migration could not have happened in the Solar System, as per the leading models of its early evolution, because it would have led to a drastically different layout than the one we know today.