Why do you think the light aircraft accident rate was 4 times worse in 1950's compared to today ?
You tell me. No idea of the reliability of statistics of that era. Met reports not as reliable maybe? Lack of sophisticated ATC radar coverage? Flying instructors were mostly experienced ex wartime pilots as against the 200 hour newbie instructors nowadays, so that wasn't a problem.
Whatever the case you quote, if true, there is no correlation between an experienced instructor sending a student on his first solo under 10 hours and an inexperienced instructor taking 20 hours.