The 400 ppm announcement was wrong but the measurements of CO2 there are long-settled science, see http://www.esrl.noaa.gov/gmd/ccgg/about/co2_measurements.html for example. There are variations no matter where in the world CO2 is measured, mostly from diurnal and annual vegetation and temperature fluctuations, mainly due to plant respiration. For example CO2 can spike at night as plants release CO2 or slowly rise in the fall as leaves decay and release CO2.
Measuring on top of a volcano that releases CO2 may seen problematic, but the contamination from the volcano is detectable as the prevailing winds on Mauna Loa shift from the trade wind direction (E) to other directions. The idea that there is way to sample volcanic CO2 and produce the steady rise shown is easily debunked. There are independent studies showing the same rise around the world with different annual variations and noise, but a matching slope.
There is no such thing as “long-settled science”. It’s like thinking that “credible government” exists.
It’s interesting that the atmosphere contains ~ 20ppm of some old Dodge Cars.