One of the reasons they cite for distro theming being bad is that it that it makes applications unusable. I think that can be validly summarized as "theming doesn't work".
Theming literally can break the UI in unexpected ways, however if often doesn’t. But if one manages the theme themselves then they are able to manage those edge cases themselves. If a distro does it then they can’t guarantee the end users are even aware that the unexpected UI is the fault of the distro.
That point was clearly laid out in the letter too. If people had bothered to read it.
