It's only a problem if you want it to run "natively" without any Linux components, which would render most of the ecosystem useless to you - it is my impression that the porters were originally trying port not just the CLI tool but also the container runtime. Every other non-Linux platform with Docker support doesn't port the runtime, it ports the CLI tool and runs the runtime in a VM - this may be because the effort was done by the folks behind Docker...The problem with that is that Docker relies on Linux only kernel features like name spaces (were supposed to be introduced in FreeBSD 12, did that happen?) or control groups. So in order to get it running you've got to rewrite these portions of docker to work differently, but do the same.
The Docker CLI tool (i.e. sysutils/docker, which is (aside from patches) the same Docker CLI tool I have on my work Mac and my personal Linux desktop) does not require Linux only kernel features like name spaces. It just requires the configuration to talk to the Docker daemon which could be running on a local Linux VM or a remote Linux host.
A while ago I mostly got colima running on FreeBSD, it's basically some wrapper around qemu that creates a VM that runs the Docker daemon. The bit that stopped me going any further was how old sysutils/docker was and how much patching was required. If sysutils/docker was up-to-date and worked then I think those that wanted it could have Docker on FreeBSD in the same way that macOS and Windows has Docker.
And I keep saying Docker but mean to say "Linux container", because Docker is just a particular implementation these days.
Slightly off topic, but personally I'm quite excited to see where the runj project goes.