Equation reference with colon in label does not work even if documented

[cderv]

---
title: test
output: bookdown::html_document2
---

\begin{equation} 
  f\left(k\right) = \binom{n}{k} p^k\left(1-p\right)^{n-k}
  (\#eq:binom:1)
\end{equation} 

See \@ref(eq:binom:1)

renders at

But our doc says : in labels is supported
https://bookdown.org/yihui/bookdown/markdown-extensions-by-bookdown.html#equations

Equation labels must start with the prefix eq: in bookdown. All labels in bookdown must only contain alphanumeric characters, :, -, and/or /.