I was wondering this too. One explanation could be that whoever sent the message to scare her (if that was their intention) knew her well and knew she used LinkedIn for flirting. People having affairs often end up having their texts and FB messages monitored by their spouses after getting caught. It's possible MB used LinkedIn for flirting because it was more private/secret. If I was suspicious my spouse was cheating, or they had cheated on me in the past and I wanted to see if it was happening again, I wouldn't even think to spy on their LinkedIn because it seems so benign and like a very unlikely way to flirt/sext. I'd be focused on FB, texts, seeing if they'd downloaded tinder, etc. Plus, texts and FB messenger show up as notifications on your phone in an obvious way, but LinkedIn is easy to shut off the notifications without anyone thinking anything of it. So let's say MB used LinkedIn for flirting to avoid getting caught by BB or by someone else she was dating on the side who looked at her phone when they were together. Only someone who already knew she did so would think to use LinkedIn to send her a creepy message.
For example, CW knew she used LinkedIn in this way because that's how they communicated. Anyone he told would also know. We don't know if CW was the only one she flirted with on there. Theoretically if she also used LinkedIn to flirt with AT, and CT found out somehow, CT could then in retaliation send MB a creepy message from a fake account with the intention of scaring her. Totally speculating and could be WAY off base, just really curious about the LinkedIn thing because when I first read the warrant I thought that was so odd. Like I think of LinkedIn as the most boring of all social media accounts and the last place I'd expect to find a sexting situation happening.