"An robh an t-orainsear saor?"
Translation:Was the orange cheap?
The verb saor means to free. So yes Saor Alba does mean Free Scotland. The noun saor is a joiner or a carpenter. And the adjective can either mean free (as in no longer captive) or cheap, but not free as in without cost. At least not on its own Saor an-asgaidh is free (of charge) and thus no cost. https://learngaelic.scot/dictionary/index.jsp?abairt=saor&slang=both&wholeword=false
Every phrase is made up at some point. That does not invalidate it. But people who hear it in English, as Free Scotland just assume it is an adjective, which does not make any sense as the slogan of independence.
If saor is an adjective meaning 'free' then it would be Alba Shaor as adjectives go after nouns and Alba is feminine, like all countries. But with the saor first it is the verb meaning 'free', not the adjective. So this is an imperative to free Scotland, in the same way that the Dubliners make the demand to 'Free the People' (YouTube link)
So it makes sense as a demand that Scotland be freed, not a statement that Scotland is free.