In the cryptocurrency ecosystem space anything "tainted" is nonsense and if Monero is "tainted" but bitcoin can't be. And with that bitcoin can't be fungible. Developers have been trying to create coins that look similar like bitcoin and use it to lure people to invest in it but it doesn't work.
If we go to literal meaning of fungible that yes BTC is fungible because it can be replaced by another BTC and both are equal in worth and value but the fact that it can be replaced makes it fungible.
Now if we talk about with facts and brain then the term fungible has no place in crypto world. What I mean to say is why someone will replace a BTC by BTC there is no point to that. No coin is fungible if we come out of the literal meaning.
Even though you bring it from the literally perspective it can be fungible because "1 bitcoin is 1 bitcoin" as the Op said. There are many similar bitcoin related coins in the ecosystem but none can withstand with bitcoin because their prices are still very far below $1. Even the Monero the Op used as the example for the fungible is doing well more than those nonsense coins. And if you talk about others I will not have problem but remove bitcoin from the fungible argument.