He's always very careful with how he frames the answers to those accusations. I think it's perfectly plausible, for instance, that ABM is not making direct cash payments to HuskerOnline, LLC (or whatever the entity name actually is) in order to run ABM content. Moreover, I also think it's plausible that Callahan or HuskerOnline, LLC have no equity or other concrete financial stake in ABM.
However, none of this is necessary for Callahan to have every incentive in the world to pump up ABM, run as much content as he possibly can, etc. There are two reasons for this. First, because ABM was pushed on every football player of any note by Frost due to his relationship with Lambrecht, it ensures Callahan has exclusive or primary access to interviews and other content with football players that a large portion of the fanbase desires. It's good for his business to have such access, because it builds up his YouTube channel and can drive subscriptions to HuskerOnline.
Second, we always talk about how the local media refrains from bashing North Stadium in order to avoid losing access. Think of this relationship as the converse of that, namely, Callahan is buttering up the key power players in North Stadium to ensure he gets even better access. Moreover, because he's frequently being exposed to players for these interviews, there is more likelihood that he can get something off the record either while the mics are off or someone contacting him down the road.
So ABM doesn't have to pay Callahan a dime for him to have every incentive to go out and shovel their horseshit to all the mouthbreathers.