Nonsense...this is how the author addresses them:
Hebrew 3:1 Therefore, holy brethren, partakers of the heavenly calling, consider the Apostle and High Priest of our confession, Christ Jesus,
They are holy brethren. Partakers (the same word used in 6:14) of the calling. They've confessed Christ.
And later:
Hbr 3:12 Beware, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God; Hbr 3:13 but exhort one another daily, while it is called Today, lest any of you be hardened through the deceitfulness of sin.
Hbr 3:14 For we have become partakers of Christ if we hold the beginning of our confidence steadfast to the end,
Again persevere to the end. It was written to CHRISTIANS. It's in the CHRISTIAN bible. Who cares if they used to be Hebrews?
These words also echo the words of Paul to Timothy which you failed to address.