OK, so a non-challenge is just a flag (or lack of http, dns, etc..) that most acme clients use to basically not perform any type of challenge, skip right over that part. This is usually the case when you are using External Account Binding (EAB), and do not need to perform a challenge to show you are the owner of a domain, etc.

In the case of Sectigo/InCommon, they have an internal method of registering all the various domains with a specific ACME account, this eliminating the need to verify with a challenge.