I'm not sure why you assume the Varangians are 'full maile with daneaxes'. They've been separated from Scandanavia for almost half a millenium, during which they almost suffered total extinction from population collapse. Their religion has shifted towards a more native american animism form of Norse paganism, even. The Varangians are well-equipped, but their primary function is loyalty, not as some unstoppable juggernaut. They aren't just Danelaw vikings transposed through time by 600 years.
Secondly, these aren't native americans by OTL. By the time the first European colonies were interacting with the native tribes they were dealing with societies which had suffered monumental (up to 90%) population losses to disease. You are not dealing with that situation. These natives have also had plenty of time for knowledge to disseminate from Vinland in regards to metallurgy and even social customs.
Thirdly, the insanely heavily armored cataphract had kind of gone extinct by the Fall of Constantinople, having been replaced by western mercenary cavalry. But even allowing for 'just' armored cavalry, it's entirely understandable for a commander to not commit cavalry to a battle when breaking through the enemy line gives you only a couple seconds of riding before going straight into a river. Next to a large native army almost exclusively using spears. If there hadn't been such a major numerical disadvantage then Theophilos could have commited his cavalry much more readily because he could have been able to actually control the battlefield more effectively.
Theophilos was sent into a battle at less than full readiness against a skilled general. Nullifying your advantages to leverage theirs isn't you putting 'strength against weakness'. Quite the opposite. You are not the Komenian Byzantine Empire. You aren't even the Empire of Nikaea.