Sierra's public, stable release was on September 20th, 2016. Xcode 8's stable, public release was September 13th, 2016. When Sierra was in beta, a beta version of Xcode 8 was also available. Apple never released an Xcode 7 version specifically for Sierra.
On El Capitan you will receive a warning but not a failure so you can use it but we never support the non-latest version of Xcode on an operating system.
Changing the wording to "forces" would be wrong. Note it already says "supports" which means if you are not using the latest version: you cannot expect support i.e. help when it doesn't work the way you want it to.
A gentle reminder that Homebrew is run by volunteers in their free time. We are limited by our time and financial resources in what configurations we can effectively support.