Disregarding genuine differences between cultures and intentially treating those cultures as identical is racist. Turns out he did come around later in life - https://www.thenational.scot/news/23716840.george-orwells-an...
Just for the record, I don't think that's a widely used definition for "racism". Also, the linked article does not really support the view that he was "disregarding genuine differences"; if anything, his prejudice made him see differences where there were none (or misattribute traits that he notice on some individual to a whole population). The article speak about his contempt for the gaelic language though, which I guess is what annoys you the most? But then the article also states that he changed his mind on that.
Also, what's the downside of treating different cultures identical, aside from potentially offending people? As opposed to other kind of racism, where other people are treated as lesser subhumans that ultimately led to slavery. Why are both casually referred to racism when the other has more far-reaching consequences.
Let's be serious, I'm not saying Orwell would be out enslaving Scots given the chance, but saying Scottish culture IS English culture is intentionally erasing a culture and is a racist statement, not unlike the common view that all of Africa is culturally homogenous.