Since 2019, the package builds sequentially:

embree (3.6.1+dfsg-2) unstable; urgency=medium

    * debian/rules: parallel builds disabled


While debugging that package, I saw it build sequentially. Maybe a
leftover from some years ago?  Seems to build with 16 cpus in parallel
on my laptop. And if memory constraints are an issue, there seem to be
new options for that in debhelper.