> Yes it does. The Xcode GUI only allows one build at a time, but you can run multiple `xcodebuild` command line builds using the same or different versions of Xcode in parallel, even under the same macOS user.

This isn't true. "xcode-select" forces you to set a system-wide Xcode version. If you have a job that uses one version and a concurrent job selects another version, it will potentially break the first job.