I noticed when using this, it correctly disables all the form fields. However, it also still allows submitting, does not hide the submit, and even pops up Saved notifications. That is undesirable behaviour, but I realized after researching this further, the documentation doesn't document disabling entire forms.
However, the Form supports it and uses CanBeDisabled which suggests maybe it was intended to be used?
Note I am working around this now by overriding getFormActions() which works well enough, but was confused about this behaviour so curious if it's a bug or just not officially supported.