The novel was rejected because it wasn't written as a juvenile. It certainly doesn't meet the usual definition of such, for one the protagonist (Johnny Rico) is a young man, not a mid-teen.
Sigh...
So, if you click on the note in the article I quoted it takes you to another document...
Like most of his novels, it was completed in a few weeks. Although written as a juvenile novel for Scribner’s, following his other immensely suc- cessful novels in that series, it was rejected and was eventually published as an adult novel by G.P. Putnam’s Sons.
Again, emphasis mine. Again, written as a juvenile... Rejected for that market, yes, but written as a juvenile.
You can believe what you like, but the facts don't support it.