Around 21:05 in the video, the build maker mentions keeping both a .50 cal hunting rifle and the .308 combat rifle in case you run low on ammo for one or the other. Other than that they did not mention which of the two is better. They simply could've been using it out of aesthetic purposes (the creator of the build does like to stick to roleplaying their character in-game) or other reasons like amount of ammo on hand when filming the video.
To compare the stats of either weapons, the Combat Rifle with a .308 receiver:
Base damage: 57 (33 + 24)
Fire rate: 49 (33 + 16)
Ammo Capacity: 20 (40 with large quick eject mag)
Accuracy: 69 (70 - 1)
Weight: 14.1 (11.1 + 3)
And a Hunting Rifle with a .50 cal receiver:
Base Damage: 64 (37 + 27)
Fire Rate: 4 (3 + 1)
Ammo capacity: 5 (10 with large quick eject mag)
Weight: 12.1 (9.6 + 2.5)
Both weapons can equip the same attachments (barrel, scope, etc.), though those same attachments weigh a bit more on the Combat Rifle.
Overall the Combat Rifle offers more damage per second due to very much higher fire rate as well as a larger magazine. But if you are just going for 1-shot kills the Hunting Rifle with .50 cal receiever has a higher base damage. I do not know for sure the AP cost for the weapons but generally higher rate of fire weapons have lower costs, so the Combat Rifle might be more efficient for heavy V.A.T.S. usage.