Kay's interest in spreadsheets wasn't about functional programming, it was about interactive and dynamic computation. I have a pdf of the 1984 Scientific American article that Wikipedia is quoting from. It does include the phrase "value rule"—by which he simply meant what we would call a spreadsheet formula—but I'm pretty sure it makes no argument about functional programming (it's all images so I can't search to be sure). If you'd like a copy, email me. It's a pretty neat article, ahead of its time as one would expect from Alan Kay.