The actual first paragraph states:

> This page collects common comments made during reviews of Go code, so that a single detailed explanation can be referred to by shorthands. This is a laundry list of common style issues, not a comprehensive style guide.

The document does not assert that you must not return interfaces or that it is incorrect to return interfaces. It only indicates that returning interfaces at inappropriate times has been a recurring issue found during code review. Sometimes returning an interface truly is the right choice, but when it isn't...

Like most adages in programming, the aforementioned tenet holds validity in many cases, but, as always, "use your noggin" applies.