RFC 2119 is BCP, not standards track
You're correct that BCP is not, strictly speaking, on the
Proposed/Draft/Full Standard Track, and that RFC 2026 refers to it as not
being standards track. It is, however, treated as a variety of standard, as
opposed to the status of a historical, experimental, or informational
document. I'd suggest a reading of RFC 2026, paying careful attention to
the fact that the statuses are called out in three distinct categories
(standards track, non standards track, and BCP) and the fact that BCPs are
closely allied with and compared to standards track documents (in several
places the terms used in the same sentence as clear equals), but used in
places where the growth in maturity imagined on the standards track is
difficult to make sense of.