Other users are apparently under the impression that content is OC if it is presented this way without a direct identification of the source. This commenter thought you had made the content in the post, and you could have clarified in a reply and said that somebody else made it. Alternatively, being as clear as possible in the title of a post about who made what is the most direct way to avoid that confusion.

I just went across that post again, and it seems that this technique has been discovered as long as 14 years ago: https://www.flickr.com/photos/nabii/5276801305/in/dateposted-public/

Is there a patent for techniques for Lego? As you said, the Reddit links show the original post, in which the OP actually didn't even comment on anything, so there isn't even any proof that this content is theirs. The post is not even tagged as "OC" on Reddit. I didn't label the post as "OC" on Lemmy either, so I don't know where the confusion could come from.