Can a smart monitor replace safe-sleep practices?

No, and specialist parenting subreddits are emphatic about this. r/parentsofmultiples explicitly warns that monitors like Owlet and Nanit have been associated with parents relaxing safe-sleep practices because they assume the device will alert them. Pediatric guidance (echoed across the reviewers we read) is that these monitors are supplemental peace-of-mind tools, not substitutes for back-sleeping on a firm, flat surface in a bare crib.