That's not correct, and it's also a heck of an undersell. Carter's three-year career average reb/40 was 8.1 He had 8.1 in his freshman year, 6.2 in his sophomore year (first year at Providence) and then 9.8 his junior year. That 9.8 is absolutely silly for a guard, and 8.1 over three years is as well.
Just looking at some great guard rebounders off the top of my head in college (coincidentally all played exactly two years):
Kidd: 6.2, 7.9
Westbrook: 3.8, 4.6
Rondo: 4.5, 7.9
Magic: 7.9 (per game, mpg not available before 1979), 8.4 (per 40, mpg available)
Magic is even a special case because he's like 6'8" or 6'9". Point is, that 9.8 blows them all out of the water, and his three-year average of 8.1 is significantly better than three of the four. It's true that you can't guarantee that college rebounding will translate (I'm looking at you, Thomas Robinson) but saying that Carter had one good year of rebounding is selling all three of his years, and especially his junior year, very, very short.
Just looking at some great guard rebounders off the top of my head in college (coincidentally all played exactly two years):
Kidd: 6.2, 7.9
Westbrook: 3.8, 4.6
Rondo: 4.5, 7.9
Magic: 7.9 (per game, mpg not available before 1979), 8.4 (per 40, mpg available)
Magic is even a special case because he's like 6'8" or 6'9". Point is, that 9.8 blows them all out of the water, and his three-year average of 8.1 is significantly better than three of the four. It's true that you can't guarantee that college rebounding will translate (I'm looking at you, Thomas Robinson) but saying that Carter had one good year of rebounding is selling all three of his years, and especially his junior year, very, very short.
Looking at Carter's raw numbers:
Freshman - 18.7mpg with 3.8rebs
Sophomore - 32.0mpg with 4.9rebs
Junior - 35.3mpg with 8.7rebs
My main point remains correct... his junior year was an outlier/breakout year. Those previous 2 seasons were nothing special.
Per 40 rebound numbers:
Freshman - 8.1
Sophomore - 6.2
Junior - 9.8