I have nothing but speculation, but it is an extremely well-written and precisely parsed note that operates on two levels: 1) a reasoned and clear explanation of who she is voting for and why, while taking the very non-controversial position that folks should do their own research and make up their own minds, while offering resources for voting information; and 2) red meat for Harris supporters with the Vance-trolling sign off (and picture choice).
I’m not suggesting that she is not capable of doing that herself, but to me it has the hallmarks of a professional writer.