@Anonymous
"I came upon this thread looking for the same answer. I see there is no answer here, "
That's wrong, you answered direct under the answer&solution in #7 from Steven !
And your answer is wrong because, as pendean wrote, flatten is not available in this software, also not in 2021.
You are talking about another software that doesn't help in this case. What you said: If it didn't exist, say it can't be done. Simple.
As first it was just unsure what OP have and what he want, that's usual. After clarification the answer was:
"What you are asking for is not an option in AutoCAD. You can't do it. Time to move on my friend"
What isn't wrong. Stevens way is a workaround to reach the goal, but there is no command to do it perfectly in one step.😉
Oh, 12:37 am, time to sleep