Leather was used yes, but as backing not as armor. Cuir Bouilli sources are sporadic at best (and usually renaissance and forward) and have not really been demonstrated to work effectively by experimental archaeology.
Ask any historical reenactor about the properties of leather compared to padded cloth against slashing - leather is among the worst fabrics you can wear for protection, but great for backing to protect against the gnaw of your armor, and if anything else it was used outside of the armor to protect against the elements - a raincoat.
Actually the only confirmed leather 'armor' in the archaeological record from the entire world is some kind of crocodile skin from Roman Egypt probably used more for ritualistic purposes than for warfare. I'll let you look for sources yourself, we've all been through this at one point, but I can assure you you can't find a single one, most leather armor theories stem from the 60s and have long been debunked. Not saying leather armor was never ever used (in the lack of other materials) but if it was in any way common you would think we had even a single surviving piece (especially as we have so many other leather finds like shoes and cords and such) or that it would have been mentioned even once in the historical record?
Try searching for leather armor on any search engine and only fantasy references show up. What is leather armor in Hollywood movies and role-playing games were usually made of linen, historically.
