The try/except is the preferred python way of doing things. In Python, try/except is cheap, and in the case where the try actually succeeds, you can gain a slight performance benefit. In this case, since performance is the goal, it is perfect for memoize.
I think subtle optimization should be left aside in a tutorial. Try except is a good way to handle many cases that are not exceptions, like when reaching the end of an iteration, but I never heard that it was "the preferred way" for checking the presence of a key in a dict. I believe the "if in" construct is cleaner, clearer and "pythoniker", if you don't mind the Housism.
"if in" is not the preferred way to check for the presence of a key in a dict. You should use setdefault. The thing is, you aren't searching for a key in a dict only. You're memoizing. And try/except is the most efficient way to do that. This article isn't teaching you how to program, it's teaching decorators.
