iOS In App Purchase: Will Apple reviewers test live or sandbox environment?

Thomas, you should definitely read Apple's FAQ on In-App Purchase: https://developer.apple.com/library/content/technotes/tn2413/_index.html#//apple_ref/doc/uid/DTS40016228-CH1-RECEIPTURL

Always verify your receipt first with the production URL; proceed to verify with the sandbox URL if you receive a 21007 status code. Following this approach ensures that you do not have to switch between URLs while your application is being tested or reviewed in the sandbox or is live in the App Store.

So yes, they are kinda crazy. But we knew that, didn't we?

@Jan P's answer is correct. If you're looking for a quick YES/NO answer, the specific answer from the FAQ is;

You should use the sandbox while your application is in review.

(15) What url should I use to verify my receipt ? Use the sandbox URL https://sandbox.itunes.apple.com/verifyReceipt while testing your application in the sandbox and while your application is in review.

quick answer: if your app in review, use sandbox.

What url should I use to verify my receipt? Use the sandbox URL https://sandbox.itunes.apple.com/verifyReceipt while testing your application in the sandbox and while your application is in review.

Use the production URL https://buy.itunes.apple.com/verifyReceipt once your application is live in the App Store.