@Charro6You didn't show the corresponding sentence, so there's no way to tell. But, 'themes', 'motifs' or 'idioms' can't be copyrighted. Copyright only covers the text or visual images on the page (thus on a patent, the blueprint and the patent application).
So, "similar" is fine, the precise working is NOT!
That's why I'm so careful about reading anything in the same genre as what I'm currently writing in, as it's typical to read something, remember the particular way they said something, and then use it in your work, assuming you just came up with it on your own.
That's a case of 'unintentional copyright', as it's still a valid copyright violation, and since every single school teacher, school administrator and/or publisher has the tools to detect these single line or single phrase violations, your chances are being caught are high, and they can often ruin author's entire career, thus making everyone question ALL of their works.
It may seem an excessive safeguard, yet I've seen too many famous literary figures lose everything as a result of a minor slip-up.
However, if you never intend to publish, and only post on SOL, you may be safe, yet simply posting to a site is the equivalent of publishing regarding copyright law. So 'not being noticed yet is not a legal guarantee.
And in most cases, when there are rumors of someone being 'sued' for plagiarism, that usually means the copyright victim was satisfied with either a cash-payment or the other authors chaining the suspected lines without making a major issue of it.
However, if you never file a copyright (last I checked, which costs $57 per 'work' (57 in my case)), as long as you don't charge for your work (potentially taking money otherwise due the original author), you can't be penalized financially. They can still have the work banned, yet that's a much lessor penalty.
Copyright law can be tricky, which is why I've always advised ALL writers to at least READ the copyright statute themselves, so you're at least away of the potential risks.
Note: Yeah, what Charro6 said.