You might get a clearer explanation if you ask on the Julia discourse here, but from what I know, the Julia package in the arch repository is built from source independently from what the official Julia provides from it’s releases on their website. I believe the official release would include the required dependencies.
In fact, the recommended way to install Julia is to get the tarball for your respective platform from the website, rather than install from the respective managers for your distribution (like pacman and apt). There are also tools like jill.py then help automate this process.
From personal experience, I have had no problems installing Julia from official release and pacman. But I’ve seen a number of users report problems with dependencies and/or using Julia packages.