From what I understand of mimblewimble is there are no records kept
There are no records kept in the *blockchain*. But every full node receives all the intermittent states of each transaction, and there is nothing stopping them from logging that information. It requires some credulity to think there are no interested parties who have logged Grin from the genesis block.
A self-pruning blockchain does not equate to a fungible blockchain.
Exactly! That's my understanding too. With some full nodes running you can store/trace transactions the same way as Bitcoin blockchain. Grin is not fungible/privacy coin.