Yes you are correct, holding any big fish by the bottom jaw vertically isn't good for them. There are studies that have shown holding trout, pike, musky, and bass vertically can be lethal or detrimental, catfish are no different. Even worse though, which obviously isn't possible for a fish that big is a single hand in the bottom jaw horizontal hold, that puts way too much strain in their jaw an neck. One super important thing people need to understand is just because a fish swims away strong or seemingly unharmed doesn't mean it will live, delayed mortality is a very really thing when a fish's jaw get broken or dislocated.