In their defence they openly state its not for the final product but rather to get _something_ shipped.
At the same time if you just take your MVP and copy paste it to production and don't fix it... that's the problem, not what they did for an MVP.
Personally, if I'm hacking out an MVP for something and someone wants to get nit picky ... I'm going to be kinda unhappy. It's an MVP there's going to be A TON to nit pick about, that's the nature of it.
I know that it is called "MVP", but firstly the term viability can mean different things and secondly there's no indication that the markup is implemented incorrectly. I love to check out example/demo pages from CSS frameworks and it's not uncommon to see glaring semantic errors in the HTML. People just copy everything that is presented to them and you should make sure to give them a good introduction.