As someone who is not a part of the FreeBSD community, I just wanted to chime in to say that the FreeBSD Ports people are absolutely correct here, and that the build time behavior you describe is Bad Behavior™ that is likely to get in the way of any distribution (Linux or FreeBSD or macOS or anything else) that follows basic best practices with respect to build sandboxing on their CI/CD systems, build clusters, etc., even if it can eventually be hacked around.
Building software outside of a sandbox with restricted network access is how you get lovely exports like credential scrapers in your setup.py or your NPM install hooks, perhaps running as root. Downloading your dependencies at build time from within the build system without first emitting a manifest with hashsums of what you intend to download is a huge problem for reproducibility, too.
