Do Catholics actually follow this canon? I think this might be one of those examples of an outdated canon that is only followed at the time and place it was in, not necessarily an eternal canon.
Can a Catholic confirm this?
As for Copts, there is no ritual as far as I know of circumcision. At least among the more modern Copts, this a medical practice, not a cultural one.
As far as the Coptic practice in the 15th Century, I'm not sure. It is true though, for example, males are baptized in the 40th day, and females on the 80th day. As to why, I cannot tell you except maybe some sort of Coptic interpretation of the Mosaic law preserved into the Church.
Besides, today, it seems the Catholic Church cares more about dogma than the literal acceptance of ecumenical councils, so even the idea to accept Chalcedon does not hinder the Catholic Church's offering of unity, and it seems that Coptic Catholics today seem to keep a lot of the anti-Chalcedonian saints as their own. The major difference between Coptic Catholics and Orthodox is the Petrine Primacy, the most important dogma in the Catholic Church, interestingly enough.