Marriott did! That's what has me so baffled. Marriott sent that before Sonder had even communicated with their own employees, Sonder employees learned about the situation from that Marriott email (when guests went down to the front desk and said, "what's the deal with this email I just got from Marriott?").
I got an email on the 9th, while it says Sonder by Marriott, talks about Marriott cancelling the agreement, and I initially said "Marriott sent" they did not. Sonder sent and signed the email.
I'd be really surprised if Marriott sent a mail to vacate premises where they don't hold the lease and don't have the invoicing relationship with the guest.