Build-depends for arch independent files

[Sebastian Andrzej Siewior <sebastian@breakpoint.cc>]

I have a question about packages like antelope. Its dependency line
says:

 Build-Depends: debhelper (>= 7), cdbs, default-jdk, maven-repo-helper,
  ant, docbook-xsl, xsltproc

and it is an Architecture: all package. No architecture specific files
are built here. The policy says in 7.1:

"This means that architecture restrictions must not be used in binary
relationship fields for architecture-independent packages (Architecture:
all)."

Unless I misunderstood it, it looks like a serious policy violation
since they are not listed as Build-Depends-Indep. 
Could it be true? In theory an arch-all rebuild of this package should
fail.
Am I mistaken?

Sebastian