Yes, but even-so it's now far easier to make an alternative WinForms implementation for non-Windows platforms. Mono had their own workable WinForms implementation a while ago (which was a clean-room implementation, IIRC), but now that isn't necessary.
WPF, unlike WinForms, is ostensibly fully separate from Win32 and so should be possible to port it to non-Windows platforms.
I wrote a project about a decade ago that used WPF and was disappointed to hear later that Mono wouldn’t be implementing a clone. They actually recommended Silverlight if one wanted cross-platform WPF. But WPF is just so much better than WinForms IMO, and Silverlight was on its way out then.
But they don't need to reimplement it entirely from scratch or as a clean-room implementation because:
1. WPF is now MIT-licensed.
2. Unlike WinForms, the bulk of WPF is managed-code, not a thin-wrapper over Win32.
Therefore anyone can now port WPF to Linux, macOS, etc - and they only need to port the "PresentationCore" part of WPF (i.e. the native parts that expose Direct3D drawing surfaces to WPF).
WPF, unlike WinForms, is ostensibly fully separate from Win32 and so should be possible to port it to non-Windows platforms.