I can't say whether vocabulary equates to IQ, but folks 100-200 years ago wrote (spoke?) much more eloquently and intelligently than today.
I read a lot from the 1600s to the late 1900s.
Yes, they were, after the mid 1700s, more 'eloquent'. For what that is worth.
There is value to be found in the phrase "I don't care who you are, that's funny right there". Not eloquent by some standards, but appropriate for today.
And none of those 15 year olds writing at a university level 150 years ago would stand a chance for long in today's world. There is simply too much going on. It's one thing to carefully think through a sentence structure while musing by candle-light near the fireplace during winter, and the fast-paced lifestyle of today.
You can't compare apples and oranges.
You also have to look at the percentage of seriously literate. I think it was highest around 1880-1920. I also believe it falls off on either side of that cut-off.