It's not. It is 100% correct. There is no written "Constitution" in The UK🤷🏻♂️.
Discussion
You're (clearly) confused (writes the Barrister (not currently practising) qualified in ... England and Wales); someone who has thus formally studied Constitutional Law. Vis; "In essence we don't have a WRITTEN constitution as they do in America. We just have a series of statutes WRITTEN ... ".
The British Constitution is more complicated than ' a series of statutes'.
Royal Prerogative powers for one. ;) #WarMaking etc