> The trashcan Mac Pro desktop was heavily promoted to tech journalists

They had one meeting where they told journalists they were not going to abandon the pro market space. This was when the trashcan had been out for a long time. Too long in fact.

> The intention was to signal pros to hang in there, and then the trashcan got released

No, the trashcan was released way before that meeting. One of the things discussed in the meeting was limitations of the trash can form factor.