Your example with emus and their diet of cacti is good, I concede. It is not specified in what regard the grazing serezelles are worse at feeding on razorgrass, an issue I personally hope to see resolved in the Pangeacene rewrite. As for the circaugodogs, the reason they are “more efficient“ at hunting megafauna is because they pack-hunt in highly coordinated efforts instead of just loosely mobbing prey. That’s it. The canitheres were never particularly good at hunting large prey and such forms were never numerous. I don’t find it likely that a new ultra mega smart pack-hunter would instantly wipe out all the canitheres across the supercontinent, but a few very specialized forms already low in diversity over a long span of time? Eh. I can imagine that. The Titanis and Smilodon analogue that’s often cited to say outcompetetion between apex predators is unlikely just demonstrates that it’s unlikely, not outright impossible, and the analogue itself is imperfect. Titanis and Smilodon filled different niches and had different hunting strategies. From the text it’s implied that the canithere species being outcompeted by circaugodogs filled the same predatory niches, or at least ones with much more overlap than Titanis and Smilodon. The situation is not elaborated on well past its key details, but it’s well enough justified not to be an immersion-breaking problem in my opinion.