From their page

"Built for better conversations Reach people with their email, not their phone number. Designed for focused, meaningful exchanges between managers, builders, and collaborators."

Is it using email protocols to send messages or is it using email addresses as a proxy for usernames?

The claim of a drive for better conversations is not really that accurate because better conversations rely on a more universally used app/system than presently exists. Ie, a replacement that would have to grow internationally extraordinarily quickly.

Apple figured that out... iMessage was basically a cheat code to a vast userbase almost instantly. What Apple didn't figure, however, was that iMessage's green/blue thingy that went on for so long didn't really give android/sms users fomo, but really, it just created an unneeded communication barrier. Such barriers are the exact opposite of what is needed for a communication platform to be excellent. Unfortunately, decisions counter to what may be perceived as income generating are difficult to reverse.

These sorts of apps may not be revolutionary enough I fear. I would love to adopt something like this, but Meta continue to make too many billions to let their monopoly on human communication management to be taken away that easily.