Perhaps many people were trying to figure out how to do vipassana in the late 1800s. Only four succeeded.
And how would this person know this if they weren't alive then?
They all started from descriptions in the Pali scriptures. The most detailed are in the Satipatthana Sutta, the Visuddhimagga, and the Anapanasati Sutta.
The Visuddhimagga isn't part of the Pali Canon. It's a commentary.
In the mid-1800s, these texts were revered because supposedly they showed the way to nirvana. However, the way they were practiced was for groups of monks to ritually chant the text in unison. This is like a bunch of people who don’t know what a computer is reading the manual out loud, hoping the machine will spring to life, without realizing you need to plug it in.
This makes everyone in 18th-century Southeast Asia look like morons in this regard, and they weren't. They didn't believe nibbana was achievable so long after the Buddha's parinibbana. They did know that this metaphorical "machine" needs to be "plugged in".
no one was seriously attempting to reach nirvana
Ever? Or is the author just talking about the 18th-century? Again, how would this person know?
The vipassana scriptures are vague, and they contradict each other. Proponents of different vipassana systems consider different suttas authoritative. They disagree strongly about which is most important, and how to interpret it.
How are they vague? How do they "contradict" each other? My understanding is that they don't contradict each other at all. They just lay out methods of practice that differ in their details.
It was Ajahn Mun Bhuridatta, born in 1870, who developed the Thai vipassana method.
Ajahn Mun's method was not an innovation. It was very much in line with the original teachings of the suttas. He was extremely well versed in the Canon and emphasized the full cultivation of the Noble Eightfold Path.
If you'd like to read an excellent study of modern vipassana traditions in the context of the suttas and the broader tradition, here it is: