I don't think the actual text leak is real, because Dragonflight is generic enough of a name where it could just be a lucky guess
Just tried the link and yeah, I'm seeing it too.
Dropping the link again in case people missed it: https://cdn.optimizely.com/js/8520856750.js
It's real, folks.
I don't think they will ever do 4 single-spec classes just like I don't think they would ever add a 4th spec to existing classes. Blizzard is a creature of habits and this kind of thing goes too far away from what they'd usually do. Most of the time, fans/players have good imagination and they often go beyond what we end up having. At the end of the day, Blizzard always take the easiest, simplest and most straightforward route.
For anyone still unable to see the leak, I made a short guide that should get you there if you're like me and couldn't see the .js in the sidebar:
EDIT: Missed one important bit, you obviously have to press F12 on the site before Step 2 to open the Page inspector lol.
Yeah it seems like the most plausible is just that every "leak" name was tried until someone found this one. I don't believe the leak is real, but based on possible themes and story of the next expansion, it's not that impossible to guess the name among the dozens of fake leaks.