You didn't read the verse. It said "atonement for sins"
The Lord Jesus Christ said "Unless you believe that I am He you will die in your sins"(plural). There is no atonement for sins apart from Christ.
Sirach did not understand that it is the LAW, to Honor your Mother and Father, and that it is SIN not to do so. Honoring your Mother and Father does not ATONE for SINS. Only CHRIST Alone Atones for SINS.
This is a Grave Error leading to death.
He understood it perfectly.
Unless you repent — you will die in your sins.
Yes, Christ died for our sins, but we must do our part too.