Loren Cordain, author of The Paleo Diet has much to say about blood type diets, none of it good. http://naturalsportsmedicine.blogspot.com/2010/05/critical-examination-of-blood-type.html
Basically he says that the blood type diet isn't based upon factual information and that any diet that ends up prescribing a paleo diet for half of its readers will inevitably see some success.
So as type A+ I should do best as a vegetarian. BWAHAHAHAHAHAHAHA HAHAHAHAHAHA HAHAHAHA WAHAHAHAHAH AHAHAHAHAH EHEHEHEE. STFU.
This presentation explains nothing and is poorly formed. There is no evidence that combining foods is worse than not combining foods. Any time someone gets in front of an audience and starts blathering about stuff like this it is time to tune out unless they have references and go into specific scientific mechanisms which you can evaluate for yourself.
I'll also note that these sorts of beliefs tend to flourish by means of confirmation bias. Every time someone reads a book they take the message in and absolutely hate contradictory evidence (myself included) because it makes them feel like they wasted time and money in something bunk. So all evidence that affirms the message is emphasized in their mind and all that contradicts it is dismissed and forgotten about.