Although I agree with the actual grading, NGC is a little strange in giving "AU" for an EF ancient coin.
Ancient coins generally are graded in this level of condition in EF or 'As Struck' (depending on condition with qualifiers), when it comes to condition.
There are many detracting minting problems that affect ancient coins, including these:
poorly shaped flans
flan splits
incomplete design detail
incomplete legends
poor striking
poorly executed design.
'EF' and 'AU' (Sheldon) can apply well to machine struck coins, but unfortunately, some or all of the problems mentioned can affect the grade of ancient coins. The Sheldon system does NOT lend itself well to the grading of ancient coins. In my opinion, the Sheldon system works best WITHOUT verbal qualifiers.
I say 'unfortunately', because if there are any problems that affect the grade, they should ALWAYS be mentioned in the seller's description, if these problems are present, and thus verbal qualified grading is needed, unlike the Sheldon system for machine struck coins.
At least, I applaud NCG for grading ancient coins, when most
TPG's will not.
I wish it was easier, but such is not the case.
Auction catalogs of World standing give a good guide in the grading of ancient coins.
By the way, that is a very nice Seleucid tetradrachm, and I would love to have it in MY collection!