A more accurate translation states...all sins committed by mortal man are mortal sins...
"If anyone catches sight of his brother committing
a sin that does not incur death, he will ask, and God will give life to him, yes,
to those not committing sin that incurs death. There is a sin that does incur death. It is concerning that sin that I do not tell him to make request."
At
1 John 5:16, 17, John evidently refers to willful, knowing sin in speaking of “a sin that does incur death” as contrasted with one that does not. (Compare
Nu 15:30.) Where the evidence indicates such willful, knowing sin, the Christian would not pray for the one so offending. God, of course, is the final Judge as to the heart attitude of the sinner.—Compare
Jer 7:16; Mt 5:44; Ac 7:60.
https://wol.jw.org/en/wol/d/r1/lp-e/1200004132#h=50:0-51:0