That might be something that foreign speakers find easier to get right because they have to actively learn it, and the words are more different in other languages so they wouldn't confuse the concepts.

Absolutely, but in my opinion it has more to do whether you learn to speak before you know how to spell and write or vice versa. Homonyms or near homonyms like "it's" and "its" or "their", "they're" and "there" are just different things for second-language speakers. For me as a non-native speaker it's a miracle how someone could ever spell "definitely" the wrong way.

His writing lacks the typical mistakes Eastern Europeans often make when writing English prose, namely omitting the indefinite article. If you look closely he might be overcompensating, for example:

    I’ve already said about the software vulnerabilities

I'm not a native English speaker myself but isn't the article superfluous here? The next one is more obvious:

   But I made some conclusions from the Marcel’s story

Actually the article in "the software vulnerabilities" is pretty useful, since he's referring to the specific vulnerabilities that let him in to the DNC. You could potentially gather that from context, but to my native ear the article sounds better.

The second example is, as you say, an obvious mistake.

It's impossible to tell if someone is genuinely making mistakes or simulating them, but just from a surface analysis:

I don't think a native speaker would ever say "I've already said about..." - you would say "I've already mentioned/written about/discussed". "Said about" is glaringly, obviously wrong.

In the second thing you cited, I could see that being a typo / editing mistake - could have meant to say "from the Marcel story" or "from Marcel's story" and it got mixed up in editing (e.g sometimes you write a sentence one way and then paste something in and don't re-read what you wrote). But if it was intentional then yes, it's definitely wrong.

Non-native speaker here. I never say "could of" when I meant "could've", or "there" when I meant "their" or "they're". I don't understand why native speakers confuse them so much. Our brains must be wired differently because of how we learned to speak and write the language.

I'm a native speaker and I never make those mistakes.

I find it irritating when I read writing that includes those kinds of basic mistakes.

Of course, you even see those kinds of basic mistakes on the TV, especially the news tickers. I see about one per week, and I'm not even consciously looking for them.

I think it's people who have never read a lot, so they just write whatever sounds similar. Just seems lazy to me to not check, and after reading these comments from non-natives who can get it right, I now think it's super-lazy.

Spell checkers can pick up on its/it's errors