In the wikipedia page "Future of an expanding universe" it refers to the scenario of a future without proton decay.

The page talks about how processes would lead to stellar-mass cold spheres of iron, calling these objects "iron stars":

In 10¹⁵⁰⁰ years, cold fusion occurring via quantum tunnelling should make the light nuclei in ordinary matter fuse into iron-56 nuclei. Fission and alpha-particle emission should make heavy nuclei also decay to iron, leaving stellar-mass objects as cold spheres of iron, called iron stars.

Under the heading "Collapse of iron star to black hole" it then says :

Quantum tunnelling should also turn large objects into black holes. Depending on the assumptions made, the time this takes to happen can be calculated as from 10¹⁰²⁶ years to 10¹⁰⁷⁶ years. Quantum tunnelling may also make iron stars collapse into neutron stars in around 10¹⁰⁷⁶ years.

How would quantum tunnelling lead to the collapse of an iron star to a black hole?