As far as forum rules go, providing the source is enough. Even just adding quotes ([quote] or "...") without giving a source would probably be enough to avoid a ban. Contrary to popular belief, the plagiarism rule and application thereof is very lenient. It's usually fine as long as moderators can see that the user didn't intend to pass the text as their own.
It is due to the fact that bitcointalk forum is not a literature forum where writers are competing with each other in eloquence and writing, where plagiarism is strictly prohibited. However, unlike on this forum, on literature forums, there are specific and clear rules regarding what plagiarism is (the notion of plagiarism is strictly defined) and what can be considered plagiarism (if all the rules are not strictly followed by a writer, it is plagiarism).
Let us take a look at the rules regarding plagiarism, which I found on some literature forum.
See Forum Rule # 6
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6. If you are going to refer to or use content from other sites, authors, or entities, you must include a link or citation for it.
You cannot copy and paste entire articles, stories, poems, or etc. from other sites or entities as that is copyright infringement, and contributes nothing to discussion.
You should not need more than a few lines, sentences, or maybe a paragraph, to make your point in reference to the topic/discussion.
From Copyright.gov ; “Under the fair use doctrine of the U.S. copyright statute, it is permissible to use limited portions of a work including quotes, for purposes such as commentary, criticism, news reporting, and scholarly reports.”
Passing off others’ work as your own, whether unintentionally or purposefully, is copyright infringement, and the Literature Network abides by the Copyright Laws of the United States of America and of other countries where applicable.
Even if Ratimov were a writer and posted his articles on the said literature forum, he would barely be considered a plagiarist because he followed at least 2 out of 3 rules. Moreover, given that the first rule is the same as that on bitcointalk forum and Ratimov strictly followed it, he cannot be considered plagiarist.