Surely some upper air time is essential,
Yes, of course that is true.
But generally the pilot being trained has a license and hopefully upper air work would be only to get them familiar with the type they are being checked out on.
Some pilots can be checked out in a very short time.
Some pilots are so weak on all the areas of flying I am amazed they hold a license, and that includes a few licensed flight instructors I have flown with.