Never heard the term before, but I'll remember it for sure.
There are so many ingredients that are virtually indistinguishable from each other, particularly when used as a side or garnish, as to make no never mind. In fact, sometimes the differences exist only in our minds.
This is true, for instance, with one of your examples. Horticulturally, there is no such thing as a pumpkin. Indeed, in the U.S., most canned "pumpkin" is actually Hubbard squash. If you lined up 15 or 20 varieties of orange-fleshed winter squash the only difference in their flesh is the sugar content. For the typical taste buds, all of the sweeter ones are virtually the same. Thus, if you pureed butternut, flat tan, and Boer white and did a taste test I don't think many people could tell them apart.
Historically, European settlers in North America called all winter squash "pompions," and didn't differentiate between them.
Many of the mild, white, flaky fish fall into a similar state. Once they're cooked, much of the flavor depends on the flavorings used and the method of preparation, rather than their inherent flavor, and are all but impossible to tell apart by most people.
That aside, I'm a little concerned about the practice. On one hand, I understand why it's sometimes necessary to make substitutions. But, on the other, it's somewhat dishonest---unless you tell the customer. F'rinstance, in the example you used, if the scallops were prepared to the customers' liking, short of a food allergy, they probably wouldn't care whether the puree was celeraic or cauliflower.
I recently had an example of this. On the menu was a halibut dish prepared a certain way. The chef didn't like what the halibut looked like that day, so wouldn't serve it. However, he did bring in some beautiful fresh grouper, which he offered as a special. But he had the wait staff tell patrons that if they preferred, he would make the menu item using the grouper instead.
It just seems like a better approach to me.