What is fungibility?

Fungibility is what makes Monero behave just like cash. The Brookings report on CBDC design choices mentions Monero and has this to say:
The impossibility of tracing paradoxically provides clarity for transferees. It means they do not need to investigate the remote provenance of the assets they are receiving for fear that some transaction somewhere far back in the chain of title was tainted.

This is one of the chief advantages of cash: except in the most blatant cases, cash is cash and it provides a reliable payment mechanism from the recipient's perspective.

So when we talk about fungibility, we are talking about the property that results from the impossibility of tracing Monero transactions. For something to be digital cash, it needs to have private transactions at the base layer. Currently Monero is the only financial technology that fulfills this requirement.

The lack of privacy at the base layer is not a theoretical problem. It affects real people in real life. There is a growing number of confirmed cases that get documented in the Bitcoin Fungibility Graveyard. Other crypto currencies are affected by this as well.

Are crypto currencies
similar to

The only unifying aspect of crypto currencies is that they all use what some call "a blockchain". The original Bitcoin paper never uses the words "block" and "chain" in conjunction. These two words only gradually combined into a buzzword, increasingly so during the ICO craze that finally popped in 2017.

It is an accepted fact, that there is nothing fundamentally special about the data structure that gets called "a blockchain":
The hype surrounding the blockchain buzzword greatly exaggerates what blockchains are, and totally neglects the complex interplay of many critical technology components that work together to make Bitcoin secure. [...] why is a blockchain better than running an ordinary database?

This over exaggeration and misunderstanding of what a blockchain is, lead to a situation where we group software under the name "crypto currency" that has very little in common.
Monero has nothing in common with crypto currencies, besides the irrelevant factoid that Monero uses what some named "a blockchain".

Monero is the only financial technology that is serious about fungibility. Therefore it deserves its own category. Monero is not a crypto currency. Monero is digital cash.