Just speaking for me, I felt like the compromises with conventional flagships (Galaxy, Note, V20, etc) were more than I was comfy with for nearly $900, which I was prepared to spend.
I don't think the term flagship means much more than the top of the line from any given manufacturer. From that perspective, the OP3t is indeed a flagship, especially since it has a comparable SoC and other components that are in other flagships.
Now if we want to discuss who makes the best flagship, that really gets into individual preferences.