That is neither clear nor visibly true. All we publicly know is that Rashford and Garnacho were both dropped and asked to show more in training. One of them clearly complied and is starting games for us despite being up for sale. The other immediately did a private journalistic interview where he said he wanted a new challenge. What we see is that Rashford asked for the move. That was the action, not the reaction.
It may have been a reaction to the club silently but explicitly putting him up for sale and telling his camp that he was not in their plans *before* the Winter piece. We don't know, but if we had to guess that it's extremely unlikely because (a) Rashford's PR would have highlighted that immediately, (b) Amorim would not have said his being dropped is solely due to effort in training, and not talked about how we needed Rashford'd qualities instead, and (c) we see with Garnacho that being up for sale does not stop this manager from picking you.
So sure, we don't completely know, but all available info strongly seems to suggest that Rashford blindsided the club with exit talk in that Winter piece.