In the original post (“he claimed his country was being held to ransom by
threats of withdrawing aid”), his country didn't have to pay any “ransom”,
it was being threatened with having its aid withdrawn.
The dictionaries simply describe what this additional occasional use of
“ransom” is supposed to mean, so you could call it “figurative”, but if
there's no demand for výkupné, there's no “ransom”, properly speaking.
I can't find examples on Google of any serious or intelligent newspaper or
writer using “hold to ransom” when there's no demand for money, so
I consider it an incorrect usage in the original post.
In Longman's sentence “he has accused the nurses of holding the government
to ransom by threatening to strike”, there's no context, of course, but
I would expect that to mean that the nurses are threatening to strike for
higher pay, so demanding money (“výkupné”) is the aim,
and I consider that example correct.