The current draft lists audio/opus as media type for "OPUS audio using OGG container" and references RFC 7845 ("Ogg Encapsulation for the Opus Audio Codec").

While RFC 7845 is the correct reference and indeed defines how Opus audio is encapsulated inside an Ogg container, it does not define audio/opus as a media type for it. Instead, Section 9 of RFC 7845 recommends audio/ogg:

An "Ogg Opus file" consists of one or more sequentially multiplexed
segments, each containing exactly one Ogg Opus stream. The
RECOMMENDED mime-type for Ogg Opus files is "audio/ogg".

Moreover, it even gives the following example:

If more specificity is desired, one MAY indicate the presence of Opus
streams using the codecs parameter defined in [RFC6381] and
[RFC5334], e.g.,

audio/ogg; codecs=opus

for an Ogg Opus file.

The RFC which does define audio/opus is RFC 7587 ("RTP Payload Format for the Opus Speech and Audio Codec"), but AFAIK this media type is for the RTP payload format, which is not Ogg encapsulated and not what one would typically find in an .opus file.

In other words, Section 3.2 of the current draft ("Core media types") lists the wrong media type for Opus audio.