Couldn't could they?

“Oxford back from 2-0 down to 2-2 at Boro. Couldn’t could they? #FACup

–Never met anything like that before. They call it multiple modals, and provide a host of examples here:Multiple modals | Yale Grammatical Diversity Project: English in North America
Too bad that lacks explanation as to why people go for that convoluted style.

I think the question in the original implied the future events (=could they go even further and win it?)
But again, why?

Sounds like a sort of yaboo comment - along the lines of ‘just couldn’t manage it, could they?’

I’ve never come across anything like that in my life in the UK, Eugene. (I don’t live in the Southern Midlands, but I have spent time there. I didn’t hear it in the US either.)
Maybe it’s not as common as that article indicates.

Your assessment of the meaning is correct for me:
(They) couldn’t (actually win the cup), could they?