In English, the noun describing the nationality is also an adjective describing belonging to, or affiliation with, that nationality. E.g. "An American is driving an American car".
In Russian, this is not the case - they are different words, sharing the same root. Some examples (noun - adjective):
US: Amerikanets - Amerikanskiy EN: Anglichanin - Angliyskiy DE: Nemets - Nemetskiy AR: Arab - Arabskiy CN: Kitaets - Kitayskiy
There's one and only one exception, and that, ironically, is the word for "Russian": "russkiy". It's the same for both the noun and the adjective, and, as you can see by comparing it with the list above, morphologically it looks like an adjective. The historic explanation for that is that it originated from the time of the Varangian conquest of Eastern Slavic lands, when the population was referred as "the people of [belonging to] Rus" - "Russkie lyudi" - where Rus was the name of the Varangian tribe in question.
Anyway, what this means is that no native Russian speaker would use the word "Amerikanskiy" to refer to Americans. It only makes sense as an adjective in "American something". However, the addition of "-s" at the end to indicate plural unambiguously tells us that whoever wrote this, treated it as a noun. Which would make perfect sense for a native English speaker, for whom the two are naturally conflated.
And the most obvious explanation for that is that if you put the word "American" by itself into Google Translate, for example, it can't decide whether it's a noun or an adjective without context, so it has to assume one or the other. And it seems to be assuming adjective by default, so you get "Amerikanskiy" back.
Oh, and by the way, writing at as "Amerikanski", without the final "y", is also something that hints strongly that it's not a native speaker. A native speaker would likely transliterate it letter by letter, starting from Russian "Американский", yielding "Amerikanskiy". However, that final "y" is really short when spoken, which is why native English speakers often miss it entirely when transcribing.
On top of that, Polish uses "-ski" for the same words: "polski", "rosyjski", "angielski", "arabski" etc. In Polish, it's also a very common (and ethymologically related - think "of ...") ending for last names - e.g. Piłsudski. There are a lot more Poles, or at least families with Polish ancestry, in US in particular than there are Russians. As a result, Polish last names are pretty common and well-known, as is their spelling. So, that spelling is often applied to vaguely similarly looking and sounding Russian loanwords and transliterations, which also leads to dropping of that final "-y" in "-skiy".
So, definitely not Russian, and overall slightly more probable to be a native English speaker from US.
That's, of course, assuming that the wording wasn't deliberately mangled to look like fake Russian, in a double misdirection...
