One could interpret her message to Ken as all part of the spite to not want him to win. I can respect your take, but that's what it is, your take. She could always just sell her stock immediately while the value was still high and before Kendall does something to tank the stock, couldn't she?
Stock price value was pricing in the deal by that point, and premium over was being for getting control. If deal did not go through, it would have cratered pretty quickly, and she would have had to navigate insider trading laws with her spouse as CEO.
I think there are typically lock up periods after takeovers/mergers like these where the acquired aren't able to sell the stock they receive for a period of time, usually something like 90-180 days. Let alone the insider trading aspects that DQ mentioned that make it harder to sell. Now maybe Kendall couldn't tank GoJo's stock price that quickly, maybe he actually is just that incompetent. But, assuming Shiv was even thinking that way, it could have conceivably costed her a lot.
In any case, Matsson will likely axe everyone pretty quickly, in this universe that ended last night.
And to me Roman is the big winner. He's always been a sad, little man, and now instead of feeling he has to pretend to be more than that so Daddy will love him he can lean right into being a very, very rich sad, little man.