My characterization of its meaning and import is completely exact. In no way was it a "Get Out of Union for Free" card.
Your attempt to gloss over the topic notwithstanding, the Tenth Amendment still, all the gall in your craw likewise notwithstanding, reserves all powers to the States that were not granted to the federal Union.
And that, my boyo, includes the right to secede, which the Southern States duly and thoughtfully exercised.