I see one problem here: caffeine is soluble in water and a diuretic, meaning anyone attempting suicide by coffee would have a bad time doing it.

Healthy kidneys work through around a litre of blood a minute, so my guess is that, ahem, "breaking the seal" would keep caffeine levels in check.

1 gallon = 7.6 l, and anectada from Wikipedia (referenced to https://www.scientificamerican.com/article/strange-but-true-...) says 6 l in 3 hours killed a man. Half-life of caffeine is approximately 5 h, so quite similar.

1 gal is about half that (3.8 liters)

UK/imperial gallon is 4.5l US gallon is 3.78l more people used to use imperial than US but now most commonwealth economies are metric.

Fair enough, but it seems per clear that the OP was referring to US gal but the math was just off by double.

Let’s go back to the OP, which asked about coffee. A quick search shows the LD50 for coffee is about 118 cups. At 6 oz per cup, that’s roughly 21 liters. The LD50 for water is listed as 6 liters (below what you’d drink “without significant issue” btw). So someone is much more likely reach the LD50 for water well before caffeine when drinking coffee.

Are there other caffeine delivery mechanisms that differ? Of course, but that’s not what the OP asked. The question was about the toxicity of coffee. That’s why it’s not worth arguing when something like caffeine powder provides the majority of ODs. Likewise there’s going to be variation in toxicity between individuals but those numbers are intended to be generalizable numbers to a population.