i think babies are actually in fact smart enough but we as parents can give them support. my baby starts sitting after she turned 5 months. at 4 months, she has been wanting to pull herself up so i only gave her support and she wanted to sit so much so i let her sit on my lap, her inflatable chair or her high chair.
I didn’t train my baby, just assisting her during playtime, in a relaxing manner. She’s able to sit unassisted when she turned 7 months old.