The guy is the one flagging the substance of his point, doesn't really matter who he is, it's the content.
Just wondered who he is. I've never heard of him
Why do you find the need to label people as 'remainer' when you have no idea? It's rather decisive and antagonistic.
A remainer is somebody who voted to remain. A leaver is somebody who voted to leave. How else am I to refer to somebody who supports remain? I haven't got a clue how you can find it antagonistic. Decisive maybe. How would you prefer me to refer to you.?
As regards, how can I be so sure this person is "somebody whose preference would be to remain in the EU". That's like me asking you how you can be so sure that somebody who says 52%, we won, get over it, is a leaver.