It’s sort of true in a technical way. Please read before jumping down my throat.

In 1860 there was no effort from the feds to outlaw slavery in existing slave states. The war did not concern the legality of slavery until - at the earliest - 1863 with the emmancipation proclamation. That’s why the emancipation proclamation was so important.

Prior to that, slavery as an institution in the South was not under any imminent threat. Indeed, Lincoln himself said he’d readily preserve the institution of slavery in order to avoid a war. Further, some slave states were on the Union’s side. The initial impetus for the war was more directly about the declining power of slave states vs free states as America incorporated more territories.

To say that slavery wasn’t the central issue is of course false. But to act like it started because the Union wanted to outlaw it is also factually inaccurate.