That is not true, cryptocurrencies do not necessarily have to be decentralised.

Although I suppose that depends on your definition, but then you would be disqualifying something like XRP from being a cryptocurrency. Which people generally agree is.

It's not an oxymoron. The first ever cryptocurrency was a centralized cryptocurrency. While the centralized aspect was the reason why it failed, the old systems from decades ago were indeed cryptocurrencies. Look up David Chaum.

Yes it's an oxymoron. EOT