Ecash on each fedimint or ecash on each cashu is not fungible either. Lightning is the interoperability.
on the other hand, the fact that they all support lightning deposits and withdrawals is probably close enough to fungibility
you can take your ecash out of a fedimint via lightning and deposit directly into a cashu mint in the same transaction
it's "technically" not the same as fungibility but at this stage it sure feels like it
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