Whereas the oak tree is still there regardless of whether we look at it, talk about it, call it different names. With the oak tree, the specific configuration of atoms matters. With Bitcoin, the agreement of people matters.
Hence, we have the power to mutate Bitcoin with the power of thought and agreement, in a way we cannot with the oak tree.
Notably, this has happened before (P2SH and some bug fixes come to mind). People agreed that Bitcoin should be changed, and viola, Bitcoin was changed, without anyone supposing we're now using an alt.
If I trim a branch off of an oak tree I've changed its specific configuration of atoms, but it's still an oak tree. Moreover, it's still the same oak tree as it was before.
If I set the tree on fire, the ashes which result are not an oak tree any more.
Clearly not all changes are equal, so it's not correct to say sat that since we've made minor alterations to the Bitcoin software over time that means Bitcoin has no unalterable definition.
There are characteristic of Bitcoin that, if they were not retained, would represent the "setting on fire" type of change as opposed to "pruning the tree" type of change.