Is the exchange built on the Lightning network or is it only a derivatives exchange that accepts transactions using the Lightning network? The article was not clear on this.
It's built directly on the Lightning Network:
LN Markets is a Lightning Network application for derivatives trading
No email or registration, connect your funds instantly with your Lightning Wallet (Phoenix, Blue Wallet, Joule, etc.)
Leverage your satoshis in a trust-minimized environment
This is just the beginning: the Lightning Network will be used as a settlement rail by all leading financial institutions
The above announcement (from March though lol) also has way more details about how it works, so everyone who's curious should give it a read.