Is watermarking the same as DRM?

No — they're different ideas:

• DRM (Digital Rights Management) tries to technically prevent copying, by encrypting or locking a file. It's heavy-handed, annoys legitimate readers, and reviewers often can't open DRM'd files in their reading apps.
• Watermarking doesn't lock anything — it marks each copy (with the reviewer's name and a traceable identifier) so that if a copy leaks, you can tell whose it was. It's deterrence, not a lock.

We use watermarking on purpose: reviewers get a normal, readable file, and casual sharing stops feeling casual. See Can a watermark prevent all copying?