Clearing some things up in this thread with some simple math and reading:
Ceylon indeed does have coumarin, but in VERY LOW amounts compared to cassia.
There is no evidence that supports @Tark’s statement of low amounts of accumulation of coumarin to be dangerous. As @horsfield said, if it could build up to dangerous levels in the body, then we shouldn’t consume cinnamon on any regular basis, ever. That is why there is a TDI - tolerable daily intake - that is to say, on average we do not want to go over 0.1mg/kg of our body weight in coumarin.
As far as the website’s conclusion, that is a correct conclusion based on the very article that @Tark linked, and there is no “but”. Let’s look at some numbers from the article:
Assuming a modest body weight of 150lbs, or 68kg, a person’s coumarin consumption is not to exceed 6.8mg per day.
From the article, in Table 1, looking at the coumarin column of the C. verum (Ceylon) cinnamon we can see that the average coumarin content is about .015g/kg, or 15mg/kg. That means for a person weighing 150lbs, one would have to consume about 0.45kg or 1lb of cinnamon in order to reach the TDI. That is a TON of cinnamon.
Looking at the most prevalent cinnamon imported into the US, C. burmannii (90% of the imports into the US during the last 5 years as stated in the article), we have an average coumarin content of about ~6g of coumarin per 1kg of cinnamon. Note that this number more highly variable, but the low/max as reported in their samples is 2.14 and 9.3g, respectively. This means that for a person weighing 150lbs, one would have to consume about 0.00113kg or 0.0025lbs to reach the TDI, which is much more likely.
Personally, I use 1 teaspoon of cinnamon per day for my Soylent. According to the calculator here:
That is about 0.0058lbs of cinnamon per day, a bit more than double what the TDI would be if I were using Cassia cinnamon.
TLDR: Based on current evidence, Ceylon cinnamon is safe for consumption as a daily additive to Soylent, while Cassia cinnamon is not, just as most of the voices in this thread have stated previously.