I don't think this statement by Sielecki is controversial at all. In fact, taken literally, it is a truism.
What he literally says is that there is no reason to read old books instead of newer ones (which implies there is nothing valuable in old books that is not covered in new books). He doesn't even say it's inefficient for improvement or even bad for your chess to read old books (something that I think would even be warranted to a certain degree, as a lot of analysis of the pre-engine era is flawed, knowledge and technique wasn't as refined, etc.).
He says "Old books are not better than new books". How could anyone disagree. But I think the stronger version "Old books are worse than new books" is true as well. This is where a debate would start.
What he literally says is that there is no reason to read old books instead of newer ones (which implies there is nothing valuable in old books that is not covered in new books). He doesn't even say it's inefficient for improvement or even bad for your chess to read old books (something that I think would even be warranted to a certain degree, as a lot of analysis of the pre-engine era is flawed, knowledge and technique wasn't as refined, etc.).
He says "Old books are not better than new books". How could anyone disagree. But I think the stronger version "Old books are worse than new books" is true as well. This is where a debate would start.