They're not. Better Auth is only 'maintaining' Auth.js to push people towards Better Auth.
Then a replacement to zombified Auth.js pops up, but this time he's early so I would take bets on him having a slice. Use your closeness (via having hired the lead dev) to facilitate "absorbing" (read: erasing) the last dregs of Auth.js, successfully replacing it with a duopoly you've invested in both sides of!
Bonus: Having raised (which you helped with) they can't undercut Clerk on some shoestring budget.. they'll fail!
The prophecy continues :) https://news.ycombinator.com/item?id=40321997