It is (as the Wikipedia article points out). It's basically equivalent to storing passwords hashed without a salt, it makes makes it obvious when a password is reused, or in this case the same file stored twice.

That means that the provider always knows if a file is shared across multiple users (even if they don't know what the file is) and given a cleartext file they can always check if somebody has it stored on the service. It's not ideal at all if you want good privacy.

SpiderOak is explicitly supposed not to do that[1] to avoid these issues, however I refuse to recommend them until they finally decide to release an open source client and 2FA support so caveat emptor.

Anyway, in this case it's irrelevant because Apple has access to the keys anyway, it's only supposed to prevent the third party (Google Cloud in this case) from having access to the files.

[1] https://spideroak.com/resources/encryption-white-paper in the "Data Deduplication" section.