This is true. The Framers were born into a world culture in which slavery was accepted as a way of life almost everywhere. Our own Native Americans practiced it long before the first settlers arrived at Plymouth Rock in the early 17th Century. And it was early in the 17th Century that the British delivered the first black slaves to their American colonies.
By the time the Framers signed the Declaration of Independence from British rule in 1776, the Revolutionary War was initiated, and work was beginning on a U.S. Constitution, the Framers to a man had come to believe slavery was immoral and debated how it should end. Slavery was still common throughout the world at that time. It would be 1833 before Canada abolished slavery. Mexico did so in 1837. Most South Americans countries had done so from the mid to late 19th Century.
Had the much richer northern states not treated the southern states as unwanted step children, it is almost certain the U.S. would have abolished slavery peacefully and without war and in a way much less damaging to the slaves themselves. But that's another story in history.
The Founders were tasked with writing a Constitution that would enable a strong, free, secure, prosperous America with choices, options, opportunity for all but had to accommodate points of view in 13 different sovereign states in order to knit them into one strong functioning country. That made it impossible to ban slavery in the original Constitution but the new government did ban the import of any new slaves to America and no states rising out of existing American territories would be allowed to be slave states.
I am pretty sure they figured that the slave states that existed would eventually abolish slavery or a Constitutional amendment would pass that would accomplish that.