That is factually misleading. First from U.S. Tariff Rates - Ratio of Import Duties to Values: 1821-1996.
"In its first year of operation, the Morrill Tariff increased the effective rate collected on dutiable imports by approximately 70%. In 1860 American tariff rates were among the lowest in the world and also at historical lows by 19th century standards, the average rate for 1857 through 1860 being around 17% overall (ad valorem), or 21% on dutiable items only. The Morrill Tariff immediately raised these averages to about 26% overall or 36% on dutiable items,
Here from Taussig and Adams:
“The bill immediately raised the average tariff rate from about 15 percent (according to Frank Taussig in Tariff History of the United States) to 37.5 percent, but with a greatly expanded list of covered items. The tax burden tripled.”
First of all, the 15 percent you quote above is the same number I've reported for pre-Morrill overall average rates.
So it appears we agree on that.
The question then is: how much did Morrill raise tariffs from the low of 15%.
The answer is: which Morrill rate are we talking about?
And that is important, because so long as secession state representatives remained in Congress, the original proposed Morrill bill rates were relatively modest, raising the average from 15% to around 22%, which was still well within past historical ranges.
Remember, worst case was the 1830s "tariff of abominations", around 35% overall average.
The original Morrill proposal came nowhere near that average.
But after secession state representatives walked out of Congress, the final Morrill law of March 1861 was somewhat higher and soon followed, in September 1861, by a Second Morrill law which was much higher yet.
Regardless, neither the First Morrill law of March 1861 nor the Second higher Morrill tariff of September 1861 were subjects of debate or reasons for South Carolina's secession in December 1860.