once two satoshis are spent from two different inputs to the same output they are indistinguishable.
Those "two satoshis" may be indistinguishable, but they are inexorably linked by their common tx. Hence, the anti-taint cottage industry of mixers and tumblers and shufflers and Darkcoins (lol). And (more significantly) the booming trade in blockchain analytics.
Let's set aside considerations of fungibility at the recipient level (maybe I'm sending BTC to MPEX, where they don't GAF where my BTC has been). There is still a privacy-driven fungibility issue for the sender.
IE, a confidential transaction is not, from my sender POV, equal to a public one. Maybe I don't want my ex-wife/IRS/boss to know where I'm sending BTC. The public and private tx are not interchangeable, and that's why people pay proportionally extra (directly and in the form of TTP risk) for fresh coins and mixed/joined/tumbled/shuffled ones.
I do not disagree that there is a privacy issue and I obviously support any attempts at improving this aspect.
The whole point of my intervention is this thread is that fungibility and privacy are two different concepts that are too often conflated.
Bitcoin, fundamentally, is absolutely fungible. Perfectly so in fact. Two satoshis are exactly alike much like two atoms of gold are indistinguishable and can be perfectly substituted.
I can taint gold and make it somewhat traceable using radioactivity. That doesn't make gold non-fungible but only traceable to a certain extent.
TLDR; When people speak of Bitcoin's fungibility "issue" they really are referring to a privacy/traceability one.