Believe me, I'm not suggesting that that diet doesn't work for you. What I'm suggesting, is that despite the science surrounding a particular diet, we can't make conclusive prescriptions and say "diet X" is the best for the general population. Many of these other diets have just as many studies and scientific support for them.
I've been on this road for close to 30 years. I've seen all the doctors from Chiropractors to Naturalpaths to Family docs to GI Specialists attached to pretty big universities, each of them has different ideas on what to try and what will work and here I am 30 years later still "not fixed". I've tried pretty much the majority of the common diets, some of helped, one set me back years (master cleanse - DON'T try that one lol).
What is working best for me at the moment is low FODMAPs. Which is in direct conflict with the blood type diet. I'm type O so according to blood type I should be eating garlic, onions, broccoli, beans; and I can say without a doubt that just those 4 foods alone are enough to wreak havoc in me, despite even being healthy (brocolli?). So in my case it's not the food group (vegetable/protein), it's the type of carb that the vegetable gets broken down into. If I don't take digestive enzymes, I'm done. Part of those enzmes are ACV (apple cider vinegar) which is basically increasing my stomach acid and again, that's a major requirement for me especially while eating protein. The blood type diet suggests I should have enough, but I can say from years of experimenting that that is not the case.
Another interesting bit; in my case, it's not only the food, but when they're eat/combined together. I can eat steak, and I can eat plain rice, but throwing those 2 together (even reducing the quantities) is when bad things happen. That's because both those very simple ingredients require different enzymes and it's much harder on my system to ask it to digest carbs with proteins, or carbs with fats. I can eat proteins with fats though. The blood type diet doesn't (to my knowledge) say much about that, so that's one area where it's lacking for a start.
I also have to question the "high stomach acid" bit of that diet as many researchers are suggesting the opposite, the vast majority of us have low stomach acid (regardless of blood type). If you have acid reflux, good chance you're making it worse by taking PPI's even though initially it seems those might help (kind of like how a bandaid stops the bleeding but doesn't do anything for "why" you're bleeding in the first place).
For your comment about bread; that's correct, bread is very high carbs which gets broken down into sugar; the whiter the bread, the more quickly that happens, spiking your glucose levels and if you're (pre)diabetic that's bad news (after the hit, you have zero energy and are extremely fatigued). Reducing your intake of sugar to zero would require eliminating bread. Many of us don't need to get that strict however, some of us can replace the nutritionally vacant "white bread" with "whole grain" or "rye bread" or "Ezekial bread" etc, less sugar and more nutrition. I can't eat bread of any kind.
So basically what I'm suggesting: it's best not to fixate on a single diet and trust in it and nothing else. Many of these diets have science and studies showing how effective they are, and yet for others we do very poorly on them. I'm glad you found something that works for you, not suggesting you can't do great on it, just that it's not going to work for many people. Sometimes you need to piece bits together from various diets and see what works.