interesting read yet I do not see why they did not adjust for 'typical intake' or 'serving' in a table or a graph. It is one to thing to extract 4 g/l caffeine in a double espresso but when you drink 2 shots of each 30ml your intake is 250mg (rounding off here and there).
Compare that to a FP with 1.1g/l if you drink 2 mugs of 250ml each your intake is 550mg, if the mug size is 150 ml that is 363mg. Any chosen Volume is pretty arbitrary for most methods, likely except that for espresso.

I typed most of the numbers into a spreadsheet so that I could calculate the caffeine extraction percentage -- i.e. grams of caffeine per grams of coffee grounds. The data were missing in many cases and very inconsistent. It's kind of a hodge-podge. The only things that really stood out were that robusta increases caffeine extraction and boiling coffee increases it too. Lots of other hints but not enough information to form solid conclusions.

I've been a lot more caffeinated lately simply because I've increased my coffee consumption by about 50%.

Adding to earlier criticisms: the summary data are presented in a highly inconsistent manner, making this literature review almost useless. The authors even note this (and other weaknesses) in the conclusion: "The authors used different amounts of coffee and water, which, among other things, influenced the final results." Sadly, general trends vanish in the diverse range of reported studies.

'Publication Pressure does increase output but not quality" a study by Marcelnl et al

In my area I see the same thing, people re-digesting old papers or doing an old study once more rather than doing something new simply or building on something potentially interesting because there is way too little money and far too much pressure to publish.
This paper IMO could have done with some peer review and editorial comments, or even better (cafeine is not that hard to analyze) have done the experiment using a standardized batch of 2 coffees with standardized extraction methods, etc.