That was kind of a PR gimmick. Excel is declarative, but misses a lot of functional. For example, first class functions.

Can you imagine a spreadsheet with first-class functions, and possibly other functional features? What would such a thing look like?

Strictly speaking, Excel doesn't have functions at all, let alone first-class, since you can't define a new function using only cells and formulas. You have to go to VBA.

Like this: http://xllfunctional.codeplex.com/. Not easy to use at all, but it is possible. Barely.