There's a story going around social media -- enough to get reporting in Wired (and I've heard it also showed up in the NYT but I haven't seen that) -- that Miller and his wife were fucking Musk as a throuple, while bragging to everyone that he could run Musk like an asset. But then Katie Miller decided she'd prefer to be with Musk than Miller, and he left Washington with Elon instead of sticking around with Miller.
So Miller essentially tried to use his wife as a honeypot to steer Elon and ended up losing his wife entirely. That's how Wired characterized it.
Now, I have no idea if that story is true. It seems too good to believe. But if Stephen Miller really did get cucked by Elon, after trying to use his wife as a honeypot, my reaction is: AWESOME. OMG finally MAGA having no bottom brings me joy.