Other than the title, Hebrews 1 and 2 gave so many clues that it was written to the nation Israel.
Vs 3 and 4 are yet further evidence, as you pointed out
3How shall we escape, if we neglect so great salvation; which at the first began to be spoken by the Lord (Matthew 15:24, Matthew 10:5, Ephesians 2:11-12 only to Israel), and was confirmed unto us by them that heard him (Peter in Acts 2:5, 2:13, 2:22, 2:36 only speaking to Israel) ;
4God also bearing them witness, both with signs and wonders (Psalms 74:9, 1 Corinthians 1:22 signs and wonders are for Jews), and with divers miracles, and gifts of the Holy Ghost, according to his own will?