Atypical “hen or roo” question

I searched through the worldwide web and didn’t find much, so this is a last resort. A couple months back I bought 3 female Porcelain Belgian Barbu d’Uccle chicks as “day olds”. I’ve recently become curious of how this hatchery is able to offer a sex guarantee for most of its Bantam breeds upon hatching which led me to search online for an answer. The hatchery doesn’t give any info on this, and from what I’ve gathered through research, this specific breed isn’t able to be sexed upon hatching. So, how?? Any insight or tips are appreciated on how they do this.