The comparison to gold is primarily as store of value...
Gold is not used as a medium of exchange or currency... and in the same way bitcoin doesn't need to be used for payments... it can grow tremendously from here just with the current narrative of being the world's first and best digital store of value.
I think that besides the store of value, we also have to add that it's both mined so they compared or at the least the reason why they call bitcoin a digital gold.