Ditto Jasper and CrazyEyes, but let me see if I can state this in a way that makes it more clear.
Suppose you were writing a beginner's textbook on, say, how to build a bookcase. Presumably you would use a simple, straightforward case: a rectangular bookshelf, all square corners, capable of supporting normal weights, etc. When you're writing a textbook, you don't normally use odd-ball examples full of special cases. At least you don't start with those. Like in my "bookcase" example, you probably wouldn't use a bookshelf capable of supporting 6,000 pounds of books, with some of the books 3 meters tall, and some of them triangular and others round. You'd deal with normal sizes and weights.
Thus, when we are discussing something that is the normal, simple case, no unusual or extreme requirements, everything pretty much typical, maybe even over-simplified, we call it a "textbook example". In real life, professionals expect to at least occasionally run into problems that don't fit the standard textbook example. Then they will have to use some creativity and original thinking to solve the problem. Thus, by extension a "textbook example" is one that does not require any creativity: it's the standard case, exactly the way people having been doing this for many years.
So what the writer is saying here is that these accusations are the sort of accusations that you would find in a text -- the normal, standard, by-the-book case. Putin didn't have to be clever or original to come up with these accusation: they're the sort that propagandists have been using for hundreds of years. The idea is that he could almost have taken any textbook on "how to smear your political opponents" off the shelf and copied them out, just filling in the names of the right people or groups where it says "fill in opponent's name here".