@comment should be followed with delimiter

[zepinglee]

A valid @comment command should be followed with a white space character, a comma, or a left brace. This is because BibTeX first scans for a identifier following @ (bibtex.web#L5433) and then determines if it's a comment (bibtex.web#L5465-L5470). Thus the comment rule should be 'comment' &[ \t\n{(].

See the following LaTeX test case.

\documentclass{article}
\begin{filecontents}[noheader,overwrite]{\jobname.bib}
@comment @book{test-comment-1,
  note = {The entry type is "book".}
}

@comment@book{test-comment-2,
  note = {The entry type is "comment@book".}
}
\end{filecontents}

\begin{filecontents}[noheader,overwrite]{\jobname.bst}
ENTRY { note } { } { }
FUNCTION {default.type}
{ "type: " type$ * top$
  "key: " cite$ * top$
}
FUNCTION {book} {default.type}
FUNCTION {comment@book} {default.type}
READ
ITERATE {call.type$}
\end{filecontents}

\begin{document}
\nocite{*}
\bibliographystyle{\jobname}
\bibliography{\jobname}
\end{document}

BibTeX treats comment@book as the entry type.

This is BibTeX, Version 0.99d (TeX Live 2023)
The top-level auxiliary file: main.aux
The style file: main.bst
Database file #1: main.bib
type: book
key: test-comment-1
type: comment@book
key: test-comment-2

BTW, there are duplicated rules named comment and they can be distinguished as "implicit comment" and "comment command".