It is certainly NOT a mortal sin (perhaps one of the most ridiculous and outrageous modernist statements of recent times).
Now, people who believe that they have to kneel at Holy Communion would be better off attending the TLM.
There is yet another aspect to this which makes the modernist position here even more absurd. Progressive liberal modernists are great at accomodating their liberal friends who want to dissent on the politics of abortion. And we hear all sorts of pieties about "liberty of conscience" and how Vatican II was all about involving the laity and accomodating their opinions and conscience. Why not "liberty of conscience" on kneeling?
Why does the modernist minimalist bishop here fail to invoke the liberal hosannas and pieties on behalf of the sacred liberty of conscience of the laity? Inquiring minds would like to know... It seems a strange omission, considering no doubt that liberal Catholics who support abortion are receiving Holy Communion while standing at minimalist liturgies in his diocese. And so, if he is willing to accomodate them in their grave liberties of conscience on matters of human life, he ought to be able to accomodate those who exercise conscience merely on one of the musical chairs issues of liturgical discipline.
I find it strange that Fr. Tran even used the term "mortal sin" to condemn those who wished to kneel. I wonder how often he has used this term during his priesthood, because it's almost unheard of today.
In my opinion, he did this to intimidate those traditional Catholics at St. Mary's who would know its meaning.