Well as late as 1942, with a reference to book The Little Prince, by Antoine de Saint-Exupéry, we didn't even know the correct shape of asteroids, believing them to be round like planets. It was in 1971 that we got a look at the moons of Mars that we got a look at what an asteroid would really look like, although we are now thinking Phobos and Deimos were formed with an impacted. The best information for what your asking can come from XKCD's What If
Before the 1970s, it was common for science fiction to assume small
asteroids would be round, like planets. The Little Prince took this
a step further, imagining an asteroid as a tiny planet with gravity,
air, and a rose. There's no point in trying to critique the science
here, because (1) it's not a story about asteroids, and (2) it opens
with a parable about how foolish adults are for looking at everything
So we had we had fifteen asteroids discovered by 1851, and until 1891, all asteroids had been discovered by telescope and eye alone, at which point Max Wolf pioneered the use of astrophotography to detect asteroids. Max Wolf himself ended up credited with the discovery of 248 asteroids.
So while I don't know when Mark Twain made that quote but as far as refering to asteroid, I'll quote wikipedia for the term 'asteroid'
Traditionally, small bodies orbiting the Sun were classified as
asteroids, comets or meteoroids, with anything smaller than ten metres
across being called a meteoroid. The term "asteroid" is
ill-defined. It never had a formal definition, with the broader term
minor planet being preferred by the International Astronomical Union
from 1853 on. In 2006, the term "small Solar System body" was
introduced to cover both most minor planets and comets.
The minor planets beyond Jupiter's orbit are sometimes also called
"asteroids", especially in popular presentations. However, it is
becoming increasingly common for the term "asteroid" to be restricted
to minor planets of the inner Solar System.
Mark Twain could have been referring to quite a few objects with the unclear definition of asteroids.