Depends on what you mean by "really meant." In the legal context of justifying secession on the basis of "breech of contract", i'm pretty sure they really meant it. In terms of being the real reason they wanted to secede, I'm pretty sure that money was their real motivation.
Given the fact that it was legally impossible to take slavery away from them, concerns about loosing it are either mass delusion, or a pretext.
If they didn't want tariffs, they could have stayed in Congress and modified them.
No they couldn't. By this point in history, they were outvoted routinely. The majority liked the system where most of the taxes were paid by the Southern states, and they were going to keep voting to maintain this system. There was literally nothing the Southern states could do to get off the hook for paying for Northern protectionism and the bulk of the Federal budget.
Another stupid thing about the theory - proclaiming the rightness and goodness of slavery cost the secessionists much support abroad and closer to home.
Arguing about what is your legal right is not always popular, but if you are making a legal argument, you are concerned with what is legal first, and whether it be popular second.
The Skokie Nazis argued "First Amendment." Their speech was not popular, but their only hope for legal relief was to argue the law, not what was popular.
As Paul Craig Roberts said, if they argued secession on the basis of Tariffs, there would be no sort of legal support for this claim, but by focusing on "breech of contract", they could make a legal argument that they hoped would convince people they were legally in the right.
It didn't work, because when you have this much money involved, legality is simply inadequate to guarantee your rights. The powerful will take the money despite any legal argument you make.
Making that declaration wasn't something they benefited from. It was something they wouldn't do if they didn't have to do it. And the tariff wasn't a strong enough reason to run the risks that making slavery their cause would make for them.
You are arguing from hindsight. At the time, these people clearly didn't realize how public opinion might turn against them. They clearly had no understanding of the power of an effective propaganda machine, and they lived in a social bubble.
Their argument made sense to people from their culture, and like many people in a social bubble, they don't realize these arguments will not resonate outside the scope of people from their culture.
Modern Urban cities and Liberal cultural elite today still don't grasp that their embrace of liberal culture is not shared by the majority of Americans. This is how they were so oblivious of the electorate to nominate someone like Hillary.
They live in a cultural bubble. Everyone they talk to thinks like they do, and we "Deplorables" are an alien body of thought to them.
Same thing with the Southerners of that era. They had no grasp of how their arguments would play in Europe, and they had no grasp of how Lincoln could mount such an effective propaganda campaign against them.
Nor were the slave states "routinely outvoted." They'd been able to do very well in Congress. One election went against them and they panicked, rather than using the power they still had. Nor was free trade ever on the table. There were going to be tariffs and they were going to rise. The only question was how high. Southern states would have influence on that, if they were concerned enough about the issue to stay in Congress.
Slaveowners did live in a bubble, like everybody else, but they knew that slaveowning was not popular or approved of among those (overseas and here) whom they wished to win over to their cause. If they believed in slavery and worried about its survival they wouldn't have trouble expressing that belief, but if they really weren't worried, if it was really all about tariffs, they wouldn't have made such a big deal about slavery. They would have found ways to work an overwhelming concern about tariffs into their "broken contract" arguments (as some states did).
The secessionists wouldn't have leaned so heavily on slavery if it were not truly important to them. They knew how Europeans and Northerners felt about supporting a breakaway regime dedicated to the preservation of slavery, but they felt so strongly about establishing such a regime that they couldn't avoid saying so. And they were passionate men, not so legalistic and bloodless as you and Roberts claim. The spirit in Charleston in 1860 was revolutionary - anything but cynical or calculating or prudent.
The powerful will take the money despite any legal argument you make.
That is shameless and contemptible on your part. Tariffs were debated and passed in the same way any laws are. There was no question that they were legal and authorized by the the Constitution.
People get tired of your idiocy. Slaveowners were powerful men. So were the leaders of the secessionist movement (also slaveowners). They weren't victims or poor saps.