Fill Rate is a mathematical term. Low fill rate also can give higher monetisation at higher CPM. And even high fill rate can give insignificant monetisation if CPM is very low. Fill Rate is used to determine how the system is performing in comparison to number of requests made. That means, it only determines if there is a demand in the market for the inventory we possess.
Demand is driven by the profile of users watching the content. If a channel’s viewers are considered valuable for a particular segment of advertisers, then yes, it can be valuable. For this particular example, if the viewership numbers of the channel are not great, then the revenue will not be significant despite higher CPM. Typically, this works as per the multiplier effect. Try multiplying 10,000 viewers $10 versus 100,000 viewers $2 for an accurate estimate.