What does “she just about finished the marathon” mean?
all it means is that the narrator has a poor grasp of grammar.
Deducing anything further from that statement requires more data, either about the event being reported or about the mental viewpoint of the narrator.
From my own perspective, it is indicative of a female runner currently in a marathon and approaching the finish line but not there yet.
What about that is poor grammar?
She just about finished the marathon: finished is past tense, it’s over and she just about (almost but not quite) finished.
She has just about finished the marathon: has finished is present progressive/present perfect tense, still in progress but near to successful completion
Not pointing fingers, but only because that’s rude. 
This is not “just about” but rather “just barely.”
In Ireland, it’s “barely”.
/on the phone/
To add to the chorus, it’s certainly not a rare expression here as well. Common idiom meaning “nearly finished.” Clearly, it’s used differently in the UK, at least, as I see from this thread, but I’m just remarking about its frequency in this post. And, while it is typically used to refer to an event in progress, but I wouldn’t say it’s unusual in the past, either. Though, like you noted, typically as part of a compound sentence: “I had just about finished cleaning up the house for guests when I realized they weren’t coming this week, but next.”
I picked #1 and I’m a lifelong New Englander.
Okay, I will try to add this phrase and its most common meaning to my vocabulary, as if I were learning something in a foreign language. I’ll mentally add “to” to the phrase, and all will be fine: “I was just about TO finish cooking dinner when…”, or “She was just about TO complete the race [when something happened that prevented her completing it].”
Aha! It’s my Kelly DNA then (even if I’m only 12.5% Irish…). 