There is a clear double entendre there.

Did the French native speaker say "that what (you) wrote would be considered as an obvious double entendre by a native French speaker", or did he say that you went over the line? That’s not the same.

As a native French speaker, I believe the first quoted statement is true: there was a double entendre. However that doesn’t necessarily mean that you went over the line, especially if the recipient is aware that you’re not a native speaker, and therefore that it may have been a candid comment.

I wouldn’t lose sleep over it, but now that you know better, I would be wary of making that same statement again.

In french, « buisson » is sometimes used to talk about pubic hairs.
Moreover, « faire très bon usage » of the two green plants sounds strange to me, unless you really use the plants (such as edible plants or cannabis).
Hence, one may tend to understand your last sentence as “But I know you make good use of your sex.”.