There are two things wrong with this argument. Firstly, European knights have not suffered the same level of historical marginalisation as native American tribes. White Europeans were the dominant race at the time of their existence. By contrast, Native Americans have endured a long and ugly history of state-sanctioned racism. Therefore, the parallel with the European knight is not accurate.
A better parallel to the use of Native American mascots would be to imagine if sports teams used caricatures of black people that played on negative stereotypes of the appearances of the black population, then gave themselves names like the Washington n****rs.
This would be unequivocally racist and offensive. There are no double standards at play. Sports teams that employ Native American names and imagery do so in an offensive way against a population that has endured centuries of oppression and marginalization.
Secondly, the use of Vikings as mascots is also inoffensive because there are no Vikings still living that would take offence to this. The same is true of Roman gladiators, Ancient Egyptian mascots or mascots depicting any other extinct ethnicity or civilization. The fact that Native American people still exist means that there are people to take offence, therefore, the act becomes offensive by definition.