I’m not going to say there is no such thing as "burn in" but there is no physics that says a copper wire will change with low levels of current flowing through it for 300 hours. Hot things, such as tubes, will certainly change. Anything that glows is undergoing metalurgical changes that hopefully will mostly stabilize, hopefully for a long period of time.
But if you were a seller and had a buyer thinking about returning an item, any item, even a rock or a nail that he just bought, if you could convince him that it will get better if he sits with it for 300 hours, that will greatly reduce the risk that he will return it.
And "audiophiles" have pretty much posted all over the internet that they will believe the "burn in" theory for any and all components.
Make your own decisions but I will suggest that you don’t believe eveything you see on the internet. Make sure you read the last line:
Jerry