Here’s a reliable and well-referenced source speaking on the matter - they are speaking on the actual amount of elemental potassium delivered, not potassium chloride (emphasis mine):
Oral doses greater than 18 grams taken at one time in individuals not accustomed to high intakes may lead to severe hyperkalemia, even in those with normal kidney function (4). Hyperkalemia may also result from a shift of intracellular potassium into the circulation, which may occur with the rupture of red blood cells (hemolysis) or tissue damage (e.g., trauma or severe burns). Symptoms of hyperkalemia may include tingling of the hands and feet, muscular weakness, and temporary paralysis. The most serious complication of hyperkalemia is the development of an abnormal heart rhythm (cardiac arrhythmia), which can lead to cardiac arrest (38). The Food and Nutrition Board of the Institute of medicine did not set a tolerable upper intake level (UL) for potassium because adverse effects from high dietary intakes of potassium have not been reported in healthy individuals (4). See Drug interactions for a discussion of the medications that increase the risk of hyperkalemia.
Hyperkalemia is the state of having high enough levels of potassium in the blood that you are at risk of a cardiac event. Hypokalemia - blood levels too low - is also a problem. The body has mechanisms that regulate the levels, both in absorption (from food in the gut), and controlling potassium loss (through the kidneys/urination and sweat glands/prespiration), but the key controller is the kidneys.
18 grams taken at once is a tremendous bolus compared to taking 3.464 grams spread out over four meals - which is only 0.866 grams per meal (that’s the Soylent 1.5 amount.) However, if you need to be extremely careful for some reason, you should be aware of the potential symptoms (which you can find at the above link), and most importantly, aware of the drug interactions:
Some fairly common things have some influence on potassium metabolism, including caffeine, ibuprofen (Advil, Motrin), and pseudoephedrine (Sudafed). Personally, I am not concerned about the standard dose of potassium in Soylent (and I use potassium citrate in my DIY). I’m not concerned even while using these OTC items. But if I were experimenting with higher amounts of potassium for some ill-advised reason, I’d take a lot of care and become familiar with the whole list.
Large amounts of sodium and/or potassium are riskier than other minerals in part because sodium/potassium balance is very important in our bodies; these are the two most critical electrolytes, which are central to cellular activity and water balance. There’s a high level of sodium in the blood, compared to inside our cells; meanwhile, there’s a high level of potassium inside the cells, compared to the blood. This balance is critical for a wide variety of reasons - it’s necessary for the activities that shuttle nutrients and stuff into cells and out of cells, it’s necessary to maintain proper water pressure inside the cells (you don’t want them swelling up or going limp), it’s necessary for proper electrical conductivity for neural cells and nerves… In fact, one of the major causes of hyperkalemia (excess potassium in the blood) is a sudden die-off of cells due to some other cause… and the dead cells leak out their potassium, which gets right into the blood.
All that being said, the usual problem in our society isn’t getting an excess of potassium… it’s having an excess of sodium! For most people, we enjoy too many salty foods (or foods with “secret sodium,” like some soups and sauces)… an extra bump of potassium compensates for the extra sodium, helping the body keep things in balance. If you aren’t 100% Soylent, your off-Soylent meals are likely to be high in sodium and low in potassium, and Soylent is a good compensator. This is, in fact, a big part of the reason why Soylent is arguably low in sodium for some people - you can see all the threads about people adding salt to see discussion on that.