I think the misunderstanding is a bit of missing context from the original comment. I read it as “one of my favorite trigraphs [that I have seen]”. Their commen

I think the misunderstanding is a bit of missing context from the original comment. I read it as “one of my favorite trigraphs [that I have seen]”. Their comment didn’t make a claim to using said trigraph, just that they had seen it somewhere and thought it was interesting. Your comments seem to suggest your reading was “one of my favorite trigraphs [to use is]”, which is understandable as a valid interpretation. The whole of this chain of comments is a misunderstanding on the original comment’s ambiguity.