“the inclusion of ISX to the S&P300 list just before suspension I think the S&P DD of the co shows confidence in the ISX cred. Doesn't S&P due a thorough enough job?”
Looks like you are too young to share market. S&P is an index and inclusion or exclusion on it is solely depends on the market cap and threshold trading volume, nothing less nothing more. It’s not them to check whether s company is dodgy or not.
And if you get mixed up with S&P credit rating, it is entirely another thing.