They aren't thermo-nuclear bombs. They are simple nuclear bombs.
The earliest types of nuclear bomb essentially smacked two sub-critical masses of nuclear material together to form a super-critical mass, or compressed a sub-critical mass to a greater density, such that it was super-critical at that higher density. Then the core material produces heat faster than it can be radiated away, and eventually explodes.
If the non-fuel components of the reactor are destroyed, there may be enough nuclear material present to combine into a super-critical mass. At that point, it will go boom unless someone acts quickly enough to stop it.
It just so happens that all reactors designed to actually be built are ultra-paranoid about not turning even a completely failed reactor into a nuclear bomb. In a backyard DIY reactor that is not following a design blessed by physicists and engineers, you might get a boom. You will more likely get a big, deadly, radioactive fire that drops toxic ash on innocent bystanders, but if you melt cylindrical fuel rods in a graphite moderator, and they pool in the bottom of a spherical containment vessel as one big, connected mass...