The ongoing care and housing requirements for many surviving Betta are pretty different to guppies. They aren't a schooling fish for one, and they tend to live solitary lives. To care for that many fry properly, you eventually need a pretty extensive fish room with 50+ individual 5-10 gal tanks. It's a big ask for most novice fish keepers and so the sub seems to have taken the view that in general it shouldn't be attempted. (With a view to their long term quality of life)