Bitcoin otoh is not shiny at all outside of the context of bitcoin. So without the network actually functioning it is just a pile of bits and not shiny at all.
The network functioned for 503 days before somebody bought a pizza with bitcoins, so the bitcoins were quite shiny by then and promised to continue to be shiny for the foreseeable future.
And this is what the Regression Theorem require: the knowledge of the past allow to project a future expectation.
Before they were exchange for fun, to try if the transaction worked correctly or not, etc.
But not for a good or a service.
There was proof the network worked for a long time and the expectation it would be up for a long time. So who acquired bitcoin could use/give them to someone else in the future.
This is the part the barter have for gold. If it was exchanged before for a direct exchange it could be used after for an indirect exchange.