Your parent has a sibling who is married. Is there a specific term for the sibling's spouse's parents? Or are they still your grandparents?
As far as I'm aware, there's no specific term in English for that relationship.
They are certainly not grandparents, that term applies only to the parents of your parents.
No, there is no specific term.
We have in English grandparents (grandmother and grandfather), parents (mother and father), children (son and daughter), grandchildren (grandson and granddaughter), Aunts, uncles and cousins.
We can also add multiple "great"s to grandparents or grandchildren to add extra generations, and add "first, second, third" to cousins for further generations before a common ancestor, and "once, twice, three times" removed for cousins who are not in the same generation as us.
Oh, and there are various alternative names for grandparents and parents, such as Nanny or Pops.
Other than that, there are no other kinship terms available.
In your situation, in order to use as few words as possible, if the Uncle is the blood relation (i.e. my parent's brother) I'd say "my Aunt's parents", or if the Aunt is the blood relation I'd say "my Uncle's parents". Even if people didn't know whether it was your Aunt or Uncle that was a blood relation, it'd be fairly obvious that you wouldn't refer to your own grandparents as "my Aunt's parents".
I think you could possibly call them "Great aunt" and "Great uncle".
The dictionary has this as
an aunt of one's father or mother; sister of one's grandfather or grandmother
Let's revisit the scenario: "Your parent has a sibling who is married. Is there a specific term for the sibling's spouse's parents?"
So, my mother (Alice) has a brother (Bob), who is married to Carol. We're talking about Carol's parents, Debbie and Edward. So, the question is: does Alice call Debbie (Carol's mother) "Aunt Debbie"? I think you'd have to say "She might".
That is to say, some extended families are closer than others. In some families, some people might refer to their sister-in-law's parents as aunt and uncle, even though they're not auntie and uncle by the strict definition (one's parents' siblings).
If this is the case, then if Alice calls Debbie "Aunt Debbie", then Alice's children could call her "Great aunt Debbie", even though she's not a by-the-book great aunt.