In layman's terms, it looks like S&P is outing Moody's for not sloppy grading, but inflating population numbers (which is unethical and causes substantial price fluctuations (investment loss)).
Coin grading (or "slabbing") is professionally done and each coin is graded and assigned a number (like a security getting a CUSIP/ISN or stock symbol)
Coin collecting is pretty fun an the amateur side like I do, but you can get into some SERIOUS investment risk with the high grade, low population numbers.
A lot of us here at FR collect coins and stack "junk silver" for SHTF/LTS for barter.
Anyways, this is very pretty big news in a multi-billion dollar hobby market....about as big as news as the fake Chinese Gold coins that mimic our US Mint gold dollar and tungsten filled trading bars.
NGC certified coin
PCGS certified coin.
I collect coins and have some in the $1 or $2k range. What is known is that in order to have a $1m coin collection you just have to buy $2m worth of coins!
The only real asset of these grading services is trust and the associated reputation. If they are inflating their grading, they are doing an extreme disservice to their customers, and in fact anyone interested in numismatics. Not doing themselves any good either in the long run.