I know what you're saying but reading that chapter again, it did sound plausible, I could imagine him as a small child being so intimidated by his father, that perhaps learning to think first and slow his speech down to prevent himself getting into trouble would help, although it didn't stop the beatings, unfortunately.
Sounes does say it was one of the reasons, not the reason, so perhaps this and the possible damage done after the bleed are the main reasons.
AS well as Bukowski saying that the event altered his speech (which is the most important thing) wasn't there anyone else that biographers could contact, who also knew him before the event to verify it?