The reason for the "harder than gold" statement is a little fantastical, but I can't be too perjorative about it as I kind of agree with them.
Bitcoin is limited in quantity by a man made limit, enforced by an open access infrastructural co-operative (the mining network).
Gold is limited by the number of gold atoms in the universe.... which isn't fixed, as stars that go supernova causes the fusion of the atomic nuclei of the hydrogen/helium mix that the star constituted prior to the explosion. This process produces more gold (as well as virtually every other element in the periodic table) than existed in the universe previously.
A lot of these newly fused gold atoms will accrete into meteroites, planetoids and planets. The planet we live on received it's gold supply in that way... except for all those other meteorites and planetoids that have collided with Earth in the past, that may well have deposited some extra gold here.
Those meteorites haven't stopped forming, or hitting the earth. It may seem a little like that, but that's just a result of a little 20th century-wide luck. Furthermore, I believe it was Elon Musk that expressed an interest in developing techniques/technologies that could capture mineral rich asteroids and meteorites, for the purpose of extracting the minerals.
tl;dr The hard limit on gold is local to planet Earth.