Here's a look at North Carolinas defensive stats in 2014 and then in 2015 under Chizik.
2014 to 2015
Total defensive ranking =120th to 35th
Pts per game = 38.9 to 22.6
yards per game = 495.7 to 411.3
pass yds for the year = 3162 to 2529
rush yards for year = 2786 to 2818
Ints = 12 to 17
Seems like massive improvement to me.
Not Sure where you got your stats from, but I found completely different ones... And Chizik does NOT look like a good hire by ANY means. I cited my sources.
cfbstats.com - 2015 National
2014
Total Defense
Rush Pass Plays Total Yds Yards/Play Yards/G
120 North Carolina 3126 3346 991 6472 6.53 497.8
Scoring Defense
TD FG 1XP 2XP Safety Points Points/G
119 North Carolina 67 12 61 1 3 507 39.0
Strength of Schedule:
Note: The NCAA method uses winning percentage of opponents, but now excludes records of FCS teams, FBS Transitional teams, and bowl games. The data below includes these three items
20. North Carolina 92-62, 59.74% (1 FCS Team - Liberty)
2014 College Football Strength of Schedule - NCAA Method
2015
Total Defense
Rush Pass Plays Total Yds Yards/Play Yards/G
96 North Carolina 3463 2640 1110 6103 5.50 435.9
Scoring Defense
TD FG 1XP 2XP Safety Points Points/G
42 North Carolina 44 12 41 1 0 343 24.5
Strength of Schedule:
Note: The NCAA method uses winning percentage of opponents, but now excludes records of FCS teams, FBS Transitional teams, and bowl games. The data below includes these three items
52. North Carolina 83-70, 54.25% (2 FCS Teams North Carolina A&T / Delaware)
2015 College Football Strength of Schedule - NCAA Method