Hi.
The relevant paragraph in Policy is this one:
If build-time dependencies are specified, it must be possible to build the
package and produce working binaries on a system with only essential and
build-essential packages installed and also those required to satisfy the
build-time relationships (including any implied relationships).
Note that this is a MUST requirement in policy.
Moreover, this policy 4.2 item is a release goal:
Packages must list any packages they require to build beyond those
that are "build-essential" in the appropriate Build-Depends: fields.
Ref: 4.2
Source: https://release.debian.org/testing/rc_policy.txt
So if you want to downgrade the bug, go ahead, but as explained,
it would be wrong.
Thanks.