No, a conversion rate like that wouldn't be sustainable and we saw plenty of lucky finishing in the first few matches. It's also worth mentioning that Opta, for some bizarre reason, aren't even including blocked shots.
Just looking at the conversion rate (especially when so many shots are excluded) without anything else to provide further context, can't really tell us much.
Here are some other, more detailed numbers if anyone is interested.
Btw, Giroud's finishing in 15-16 was perfectly fine. So that argument, if we're going to limit it strictly to just finishing, is bullshit.