iOS development is not really compatible with the GPL; both the FSF and Apple agree on that. But there was nothing in macOS that was incompatible with the GPL,

iOS development is not really compatible with the GPL; both the FSF and Apple agree on that. But there was nothing in macOS that was incompatible with the GPL, and yet the FSF for a decade prohibited its maintainers from accepting macOS specific patches (So much for “freedom”). To Stallman, Apple’s “Look & Feel” lawsuit was an unforgivable transgression, but Microsoft’s various tactics to muscle out competitors were never a cause for concern.