I think you missed my point.
Here's another example of where Hebrews is not written to Christians:
Hebrews 10:26-30
26If we deliberately go on sinning after we have received the knowledge of the truth,
no further sacrifice for sins remains,
27but only a fearful expectation of judgment and of raging fire that will consume all adversaries.
28Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses.
29How much more severely do you think one deserves to be punished who has trampled on the Son of God, profaned the blood of the covenant that sanctified him, and insulted the Spirit of grace?
30For we know Him who said, “Vengeance is Mine; I will repay,” and again, “The Lord will judge His people.”
As you can see, if a Christian sins deliberately after knowing the truth, the sacrifice of Jesus still covers them. This only applies to those who sin under the old covenant and used animal sacrifices to cover their sins. If they deliberately sinned after the animal sacrifice, then the animal sacrifice was no longer able to be a valid sacrifice for their sins. They would need to do another sacrifice and would likely have to wait until next year. This is a persuasive speech, a sermon, trying to tell those under the old covenant to come to Christ asap.
Hebrews is written to a mixed audience. The above passage is written to practicing Jews.
If the above passage is to true believing Christians... God help us all.