“Another flaw in their argument is that chrmical analysis has been used since 1812”
That is incorrect. Attempts to measure CO2 content in the air were notoriously unreliable, difficult, and unsuccessful.
CO2 was not even known until around 1750.
All measurements prior to 1930 are worthless.
That is not correct. Here's 0.04% from around 1919: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1405551/pdf/jphysiol01752-0035.pdf That's of course higher than the alleged global average then which was 0.03% But there are diurnal and geographic fluctuations. You have to take a lot of readings to get a valid average.