Also, let's be honest. We're talking about a candidate that blamed the south for being uneducated and lacking in internet for his loss there. It's not exactly wrong or out of character for anyone to interpret his words here as a literal purity test or come as bitter.
This is some pathetic spin. If you're a minority in the south you are more likely to be poor due to the disenfranchisement of minorities, especially in southern states. If you are poor you are less likely to
a) afford higher-level education, meaning you will less frequently be exposed to alternative political messages such as those espoused by Sanders
b) have internet access, a medium that gives alternative political views (like the Sanders campaign) a platform that isn't dominated by traditional media outlets
So of course it makes sense that if you are living in the south, a large amount of the population is going to be generally unaware of Sanders but vastly more aware of someone like Clinton, who is practically a brand name that has been a fixture in American politics for decades, especially among people who are only exposed to politics via traditional outlets like TV and radio. "Low information voters" is a fact of life in poor states, but the attempt to make the acknowledgment of these voters some kind of smear against Sanders is ironic, coming from a camp that positions itself as "realists" as opposed to Sanders' idealism.
This bullshit of demonising literally every action taken by Sanders (and the responses in kind from the hardline Sanders camp) is far more divisive to the Democratic party than Sanders conceding with 5 months to go instead of 6.