Sorry, couldn't resist.

"re:" is a professional but maybe outdated way of responding to "regarding" something.

In musk's case here, who fucking knows, billionaires all use emails like shitty chat, and pedo billionaires all talk in code, so it could honestly be anything or even an automatic subject line fill.

I'm assuming it just means "regarding (the thing WINK WINK)"

Edit: usually it's typed something like this:

Subject: RE: birthday party
Body: make sure the dancers are only paid after they perform. Also, no peanuts, half of attendees are allergic.