Hello @keen and welcome to the forum!
I have neither LucidLink nor a Mac, so can offer only limited help, but I found recent work on space reporting:
Release notes for build #1488, release September 18, 2018
Release notes for build #1520, release October 11, 2018
That sounds like two different measurement mechanisms. Can your local storage measurer also measure the available storage at the LucidLink mount? This mentions some ways including an easy one using Quick Look. From the command line, perhaps you can run a df command, which perhaps runs statfs (see build #1488).
Duplicati.CommandLine.BackendTester.exe gives space information at the end of its test, based on DriveInfo.
As a side note, LucidLink’s use of a write cache worries me because the actual upload is delayed (till when?).