I wouldn't really call it a functional programming language, as it's quite data centric and operations are encouraged over arrays and tables. Writing functional code is often an exercise of composing functions; whereas in Excel you might compose functions, but your job as a programmer is mostly entering data and defining operations over them. Reactivity is a very important aspect of the whole thing that's missing from most functional programming languages. To reduce Excel to functional programming is to boil away its essence.

Edit: Watching more of the linked talk, it's pretty much making the point I am except some of the terms she uses are not the ones I would use. The speaker defined "functional programming" as programming without assignment statements, which if we are to accept we must then conclude that Prolog is a functional programming language.

Also insofar as she says Excel is a pure functional programming language, she means that the language has pure functions. She's not saying it has all the other features of Haskell like static typing even though her graphic kind of implied it.