It wasn't rejected for improper grammar.
I see two potential problems with your edit:
1) You used
this monospaced font for "Let's" and "Let us". Our convention is to avoid using "codeblock" text, and use either quotes or italics instead.
2) You replaced:
But as my speaking language isn't English, I want to know clear understanding for using these words.
I am asking because I am not a native speaker and have this confusion.
I don't think "have this confusion" is a good way to say this. More importantly, though, I think a reviewer felt that the "I'm not a native speaker" part was unnecessary, because that's what ELL is for.
In short, my guess is that the reviewer felt that the suggested edit added as many problems as it fixed, so it would be easier to simply make another edit altogether.
However, your edits addressed some legitimate concerns, and these might not have been noticed had you not made your edits. So, in the end, even if your edit was rejected, it was still useful for the community.