Having an imprimatur does not make it so. This catechism was written by a UGCC priest from Canada, in the English language, for Ukrainian Catholics in Canada and the U.S. It was published more than 30 years ago. The UGCC published its official catechism just in the last few years.
No, I am not saying that it does not accurately convey the faith of the UGCC. However, I stand by what I said, Eastern Catholics tend **not **to use the terminology of mortal vs. venial sin. The use of such terminology in this catechism does not disprove my assertion. After all, the UGCC is only one of 21-22 Eastern Catholic churches, and as I’ve said, this catechism by Fr. Casimir, published in 1983 (a time when Eastern Catholicism still tended to be highly Latinized), is not an official church catechism. Even if it were an official catechism of the UGCC, that would say nothing about the other other 20-21 Eastern Catholic churches.
The teaching of mortal sin vs. venial sin is universal Catholic teaching. The terminology itself is not universal. I, unlike you, actually am Eastern Catholic. I don’t think I’ve ever actually heard any Eastern Catholic actually say “mortal sin” or "venial “sin,” and I’m sure I’ve never heard an Eastern Catholic priest use the terminology. It’s simply not part of our Eastern patrimony.
We believe that all sins should be confessed. We believe that some sins are more serious than others. We believe that some sins (sins of grave matter, to use Latin terminology) are so serious that one must confess them and receive absolution prior to receiving the Holy Eucharist. This is believed by the Orthodox and Eastern Catholics, and Western Catholics. It is not necessary for Eastern Catholics to use the same theological terminology Latin Catholics use.