I don't agree at all, but there are arguments to be made both ways.
Here is a post covering some of the times in pre-COIE when a father/son relationship was showcased. Not nearly all instances are covered here, though I'm sure there are brotherly instances, too, but this is definitely evidence that it was not some idea created whole cloth in the post-COIE era. To me, it was the default until the 1970s.
I will admit, I am shaped by that I think the ideal relationship between a parent and adult child is one of equals. So when, as Dick grew up, they became friends and Dick became someone Bruce could confide in, etc., that does not switch the relationship from a parent/child dynamic to a sibling to me. Alas, that possibility is long gone and now Dick must forever be subordinate.