Soldato
Why not ask the guy who used it?
The likely answer is it seems improbable based on current knowledge that humans could affect the global average surface temperature much before then.
Absolutely, then the question would be what was the cause of the initial large rise pre mid 1700s. It's an uncomfortable question and obviously one they don't want to bother answering which is why they start in the mid 1800s. I get it, it detracts from their message which is why they do it, it just feels dishonest.