Clingman himself began his career as a Whig from a mountain district. So he was at least in theory not ill-disposed towards the party of higher tariffs early on in his career. In 1845 he lost an election because he was perceived as being insufficiently pro-slavery. As is very common in politics, he apparently resolved not to give his opponents cause to make that reproach against him, and became very proslavery and well disposed towards secession. In 1852 he left the declining Whig party and was eventually rewarded by the Democrats with a Senate seat.
Clingman's contributions to DeBow's Review (Coolies, Cuba and Emancipation, and North Carolina, Her Wealth, Resources and History reveal him to be a brutal biological racist and an imperialist. Clingman thought nothing of being published in the same volume with proslavery theorists like Henry Hughes and George Fitzhugh. It's natural that he would support tariff policies that would promote a slaveholding empire, and oppose those that hurt slaveholding interests, but he was certainly not an innocent freetrader devoid of interest in slavery expansion and racialism and provoked to rebellion by the Morrill Tariff.
An interesting argument, but also one with a very severe and glaring flaw. That speech was in January 1860. In between then and December when secession started, the Republican Party formally adopted protectionism as a central platform plank and nominated a well known protectionist as their presidential candidate. Therefore, while it is true that in January 1860 the Republican Party did not subscribe to protectionism, that changed in the summer when they enthusiastically endorsed it as a central plank.