Yes, this is all true but Sutter's comment is that the specific platforms that this specific implementation of the GSL targets results in the correct output. The platforms officially supported are:

GCC 12, 13, 14

XCode 14.3.1, 15.4

Clang 16, 17, 18

Visual Studio with MSVC VS2019, VS2022

Visual Studio with LLVM VS2019, VS2022